So, in 3rd edition (which is what I started with years ago), there was only the A, B, C... system. The problem lies in that you can't have multiple areas at the same priority level (well, you can but you only lose out).
There are five categories:
- Race
- Attributes
- Magic
- Skills
- Money
Now if you want a human cybersamurai, then race can be at E because you want a human (basically, all non-humans get bonuses of some sort and are better than humans, but you'll usually suffer pretty blatant racism because of it). Magic can also be at an E because you don't want any magic -- you're a cybered individual whose soul is already going to be split apart by machines and you'd be a crap magician anyway. So now you have the other priorities at A, B, C, and two at E, but you don't get anything by "giving up" a D category. Well, you could change the race to elf instead of human, but if human is your concept then you're just missing out.
The karma system started as an optional system in a side book and either in 4th or 5th edition was brought into the main book. You know how, after you start playing, you can buy skills and attributes with experience? Karma works a similar way -- you buy up your starting attributes/skills with karma points.
Sum to Ten basically takes the old priority system, and tries to assign a point value to each letter. So if you want an A, B, C, E, E, individual then because you don't have a D you get to give a small bump to A, B, or C. You could even go with A, A, C, E, E, for instance.