Every time you play a Mission you gain 1 XP to spend on the character you played.
Every time you Monster a Mission you gain 1 XP to spend on any character you have played at least once (or you can bank this unassigned XP for later use).
Every time you are a GM or co-GM for a Mission you gain 2 XP: 1 for running the Mission and 1 for writing up the OC Mission Report.
XP is spent on new Abilities from your Class (or on Experimental Abilities).
You may only spend XP in Ranks you have unlocked for that character.
You unlock new Ranks by spending 4 XP in Abilities up to the Rank below, and playing at least one Mission at that Rank.
At Rank 1, Rank 3 and Rank 5, you unlock a free Class Feature - this does not cost XP to access.
You may also access
General Abilities with the same rules of Rank (but have no Class or Subclass restrictions)
For example, a Valid XP Spend:
An invalid XP Spend (no XP spent on Rank 2 Enforcer, so can't access Rank 3 Enforcer):