When you first report to your facility as a new hire (obviously after you have passed the academy) you earn AG pay (Its the same I believe in 2012, only in the shown document here it has been clipped out) When you reach a certain stage of progression and certifications in training you move in sequence to the bottom of the D1,D2 and D3 pay bands for your facility level. When you fully certify you move to the bottom of the CPC pay bands. The points that it takes to reach D1,D2,(if applicable) and D3 are facility dependent. I believe at most centers D1 is A side plus 2 D sides (radar assist), D2 is All D sides, D3 is 2 R sides (the actual position that vectors) and CPC is all R sides. In an up down it may go D1 is ground, D2 is skipped and D3 attained after Local and CPC reached after radar certification. Its really just how the facility works it. Smaller facilities just don't have enough positions to justify having all 3 Develpomental pay bands.
The upper ends of the D1,2 and 3 bands serve two purposes. A) A new hire in training, lets say a D2 at a level 7. Its the end of the year, time for yearly raises. Say they get a 3% raise. They get that increase to $53,497. Now when the make D3 they go right back to the bottom of the D3 band, but they still got a yearly raise.
B) CPC at a level 5 transfers to a level 9. This one is in reality far more tricky due to the contract's rules on pay for transfers, but i'll explain a neutered version for ease of understanding as to why there is a maximum. The CPC-IT is making $54,712 (they transferred right after checkout lets say) when they step foot in their new level 9 facility. They clear the D1 goal posts. D1 as base is less than they currently make, so no raise for them. Now they make D2, suddenly the pay band shifts at its bottom to be more than they currently make, our CPC-IT now gets a raise.