[douglas]

It's a progression. After the first rule expires, the second takes effect. From Abbidon_AI_Settings.txt:

Max Ship Size Tonnage From Start 1 Amount := 510
Max Ship Size Tonnage From Start 1 Num Turns := 20
Max Ship Size Tonnage From Start 2 Amount := 610
Max Ship Size Tonnage From Start 2 Num Turns := 40
Max Ship Size Tonnage From Start 3 Amount := 0
Max Ship Size Tonnage From Start 3 Num Turns := 0

With these settings, the Abbidon will not design ships larger than 510 kt for the first 20 turns. For another 20 turns, the Abbidon will not design anything larger than 610 kt. After turn 40, the restrictions are gone. I'm not sure if the order matters or not, but I would guess that only the first one that hasn't expired is checked. You can probably make a longer series of graduated restrictions on ship sizes, but I haven't tested it.