The numbers refer to the person's "IPC" (incentive pay classification or category, I can't remember exactly). A brand new captain would be an IPC Basic (or 0), whereas someone with four years in rank would be an IPC 4. You get bumped from one IPC to the next automatically as the years pass (i.e., your IPC is not tied to performance reviews).
It used to be that there were only four IPCs per rank (i.e., 0-3), but they changed that approximately five years ago. The fact that there are so many IPCs for Captains and Majors should tell you something about the promotion blockages; I was a Captain from 1990 to 2001, when I finally figured that there was no hope of promotion (okay, I'm a bit slow) and got out.
I don't know what the current aircrew allowance is, but remember that in addition to that allowance, pilots have a significantly higher rate of pay than non-pilots.