Traveller is a science fiction game setting which was first published in the 1970's, and which has since gone on to be one of the longest running and most popular SF games.
YAGS SciFi assumes a Traveller-like campaign, partly because this is the setting it has been used to run games in before, and also because it makes sense. Traveller uses the view of science fiction from the 1970's, and doesn't get sidetracked by the revolution in computer and genetics technology that came after. The setting is nothing like the future will be, but this allows the GM and players to concentrate on human characters in a human society, rather than playing minds uploaded in AI generated virtual landscapes.
So YAGS SciFi is not Traveller, but it's similar in style.
One thing about Traveller is that it assumes combat in space takes place across millions of kilometres, with beam weapons being effective across these scales. YAGS SciFi assumes a smaller scale for spaceship combat, one which is more action orientated (but not as action and SFX focused as Star Wars for example).
By limiting combat to a few hundred kilometres, it makes smuggling operations and blasting your way out of star ports that much easier - a single patrol cruiser can no longer pick you off from high orbit as you attempt to escape to make a Jump to hyperspace. Fighters, though still limited in role, are also easier to imagine when they are fighting combat at ranges of kilometres rather than thousands of kilometres, with turns measured in seconds rather than hours.
If you've played a computer game like Eve-Online, then you'll have an idea of the sort of scales that YAGS SciFi assumes. In all, it is thought that the shorter ranges provide a more player-friendly environment, and may actually be more realistic. Focusing lasers over very long ranges is actually very hard (if you thought laser beams didn't spread out over distance, think again), especially if you want it focused enough to cut through ship hulls.
Another assumption made by YAGS SciFi, which is a departure from Traveller, is that grav technology doesn't allow anyone to fly up into orbit on a supermarket trolley. In some respects it is closer to the repulsor technology of Star Wars - grav vehicles have an operating ceiling above which they can't go.
The main reason for this is that without this limitation, there is no such thing as ground vehicles. Tanks, civilian cars and hover boards become space ships, which is a blurring that removes specialisation.
It is assumed that there are two types of 'grav' technology - thrusters which are used by spacecraft, which are big and expensive but which can provide thrust in space; and repulsors which are more like reverse tractor beams, pushing against nearby matter to provide lift. The ability of a repulsor to provide lift drops off rapidly with distance (to a very high power), so many vehicles are limited to a few tens of metres altitude. Repulsor tech is relatively cheap however, so is used in vehicles designed for use within an atmosphere (high-altitude repulsors, limited to a few tens of kilometres are available for aircraft at high tech levels).
The technology levels used here are slightly different from that used by classic Traveller. The main reason for this is to have at least one futuristic technology level clear of any magic science such as jump drives and anti-gravity.
Similar to early 21st century Earth, up until about 2025. Personal communication devices are common, though hampered by slow advances in battery technology. Nuclear fission and fossil fuels are the most abundant form of energy generation, with some solar and other renewable sources.
Large number of dense urban centres, limited space travel to near orbit and heavy use of communication and intelligence satellites.
Breakthroughs in energy storage technologies begin to make smart devices a lot smarter and more useful, and computer networks continue to take over the social aspects of people's lives. Privacy crumbles.
There may be limited colonisation of the local system, with new plasma and ion drives opening up cheaper space travel. Early fusion reactors, and growing usage of solar, wind and tidal energy sources push the energy uses of the culture rapidly upwards.
Genetic engineering leads to artificially grown food sources and the eradication of diseases amongst the wealthy. Cybernetics becomes relatively common, as does limited forms of regenerative surgery.
Fusion power becomes the dominant form of artificial energy generation, replacing fossil fuels completely. Solar energy is also ubiquitous, and provides distributed energy collection both on planets and in space. In fact, the energy consumption of TL 10 societies is such theat they often considered Type I Kardashev civilisations.
New advances in batteries enable the use of long lasting smart devices and laser weapons, though the latter aren't as efficient as slug throwers. Smart clothing, which can change basic properties (stiffness, colour, size) according to surroundings and it's wearer's physical state are available.
Intra-system space travel becomes practical on a large scale, and orbital colonies are frequently found around such cultures. Jump drives become available in the latter half of this tech level, though they are bulky and inefficient. However, they open up the possibility of interstellar colonisation.
Jump-1 drives become commonplace as the science and engineering problems are solved. Genetic engineering and medical advances allows for the complete rebuilding of human bodies, leading to life spans approaching two hundred years for the wealthy. Cryo freeze becomes possible for those who need to hibernate for long journeys, or need to await medical advances.
Particle beam weapons continue to grow in deadliness, and the better understanding of nuclear forces allow for micro nuke weapons in space and on the battlefield. This also leads to the first nuclear dampers as scientists figure out how to manipulate the basic properties of matter.
Jump-2 drives become possible and common, opening up the galaxy to rapid colonisation and trade. As nuclear damper technology improves, it leads to repulsor technology - and the first hover vehicles. This has a big effect on the use of transfer to and from a planet's surface, though the range of repulsor beams is currently limited.
Cold fusion soon follows, which allows the generation of micro-power in small devices. It's not efficient for large scale power generation, but a smart PDA with a cold fusion cell can last for weeks without external power.
Plasma weapons start to be used in ship to ship combat, which though they have a shorter range than particle beams, pack a bigger punch against larger targets.
Jump-3 drives become possible and common, and repulsor technology improves in leaps and bounds. Hover vehicles are common, and even sub-orbital craft begin to take advantage of the effects. In space, fusion drives provide another leap forward in propulsion technologies, and fusion weapons become popular on warships.
Other weapon technologies are Meson Guns, which can bypass ship armour, and Meson Screens which can protect against them.
Brain taping becomes possible, though it is considered illegal tech throughout most of the Imperium. The first AIs are also created, but they do not see widespread use. However, some of the technology trickles down to smart PDAs which enables a whole range of extra responsibility to be handled by automated devices. The Imperial nobility make it a point not to make use of this, since they view it as a toy for the lower classes who can't afford real people to perform such tasks. However, synthetic pets with limited AI are popular playthings for all classes.
Other medical advances means that everybody is beautiful, even the poor, and unhealthy life styles can be fixed with pills. Agriculture is dominated by genetically engineered crops which can grow anywhere and produce any flavour and texture desired. The eating of real meat is seen as an exotic, if not barbaric, taste.
Jump-4 drives become possible, though there are signs that there may be limits to Jump technology.
Further advances in damper fields leads to partial gravity nullification, with the ability to tweak the level of graviton interaction. Together with improved repulsor fields, floating buildings become popular on rich worlds. Inertial dampers also allow for safer space travel.
The creation of Nu Matter allows for the construction of completely artificial materials with all sorts of exotic properties. New types of armour, and also textiles, become possible. Clothing which can adapt and change to the environment becomes standard fashion.
Jump-5 drives become possible, though they never become as popular as the shorter range drives due to their larger size. Black globes protect warships, whilst anti-matter missiles and fusion beams try to rip them apart.
Rich high tech cities consist of impossibly tall buildings linked by impossibly thin walkways. Above them the estates of the nobility float on islands of rock and glass which conveniently ignore the pull of gravity. Few modern cities are paved with streets, instead lush gardens fill the spaces in between whilst civilisation goes on above.
Industrial worlds are often circled by orbital habitats, whilst the crust is mined for anything of value. Minerals are ferried to orbit in long streams of repulsor carts, and factories pull apart the very atoms and refashion them into whatever is desired. Technically, basic hydrogen can be fashioned into anything, but it is more economical to start with something as close to the desired outcome as possible.
Jump-6 drives become possible, though they are inefficient. It is theorised that this is the maximum range possible for a Jump drive. The combination of repulsors and gravity dampeners make it possible for pretty much any small or medium ship to land on planets with ease. In space, warships are protected by white globes, from behind which they use meson cannons and neutrino guns to destroy their enemies.
At the height of Imperium culture and technology, worlds are home to huge migratory arcologies which float over the landscape, modifying their form and function at the whim of the nobility.