I had a look at the CTC AQC scheme just over a year ago and went to the selection day (it was free at the time). During the sales spiel they briefly mentioned that the hold pool is tiered with the integrated bods in the top tier, people who did some component of their fATPL in the middle and the AQC only at the bottom. They were vague about the whole thing and couldn't (or wouldn't) tell me the mechanics of how the different tiers were called up for opportunities. This lack of clarity and evasiveness was a major factor in me turning them down and going elsewhere.
I would add the caveat that this was over a year ago, so things may have changed. It is however an important thing to nail down before handing over your hard earned cash.