Traveller's Aid Society: OOC
Some skills are called Cascade skills because they have several Specializations under them. Like Science. For simplicity, the game only deals with 18 Sciences, such as Biology, Physics, Chemistry, etc. When you first gain the Science skill, you have it at Rank-0; that represents the core training you need to specialize in any of the Sciences. The next time you gain Science, the basic Science skill remains at 0 but you now have to pick one of the Sciences to specialize in. So, say, you choose Physics. You have Science (Physics) at Rank-1. Get Science again, you can boost Physics to Rank-2 or add another Science to your repertoire, such as Chemistry-1.
The 18 Sciences listed are broad enough that they should cover most scientific paths, but they aren't meant to be all inclusive. If you want your character to be a Forensic Pathologist, you can choose that instead.
This applies to all the Cascade skills, though some of the others are broad enough that there aren't really any additional specializations to choose. For example, Gun Combat (slug) pretty much covers any gun that fires a projectile, including Gauss rifles, needlers, BB guns, etc.
Since it didn't happen for my character, I didn't know the Character Generator could kill your character. Silly. "But, GM, I don't want to play a dead character--al he does is lay around and rot!"