Parameter Rules

For no particular reason, all the Servants had their abilities translated into specific parameters.
It's pretty obvious just from looking at them that A is the strongest, but you might be thinking to yourself, “What on earth is all this B+ and A+ nonsense!?” Well, don't worry, because I'm going to take a moment to explain the rules behind it here.
For sake of argument, let's assume that a normal value is 1. In that case, E would be 10. And, every rank after that adds another 10, all the way up to A, which is 50.
Now, things like B+ and A+ represent the unique ability to multiply these numerical values for just an instant.
In other words, an ability ranked B+ would normally be weaker than an ability ranked A, but can momentarily exceed it by doubling its own power from 40 to 80.
Heroic Spirits with a + (plus) are rare, those with a ++ (double plus) are extraordinary, and those with a +++ (triple plus) are in a class all their own.
Also, a truly exceptional ability score that falls outside the numerical ranking scale is represented by an EX.
Though Gilgamesh's ability scores generally aren't very impressive, his EX rank Noble Phantasm parameter clearly distinguishes him from the other Heroic Spirits.

Fate/side material: Fate Encyclopedia