FFS is a seperate book from the TNE mainbook. You of course need the mainbook to understand the rules you're using, but for the most part, FFS can stand alone, as you are given things in terms of every day values, like megajoules, kiloliters, and meters. For instance, you make a pistol that has a power of 2000 joules. FFS gives you a formula that tells you what 2000 joules does in TNE terms, and to understand that, you'd need the mainbook, just like a supplement for any other game system. That FFS gives you the value in joules, though (in addition to the mass in kg and the range in meters and stuff like that) allows you to translate the item into just about any rational system. And you get the satisfaction of making something really cool (if you DO make something really cool).