It's for standby/non-revenue travel. The priority is AAC - "Non-wholly owned regional affiliate" or something like that. With United, it will be the same priority as other UAX/United Express carriers.
It's unlimited. You have to pay a $1.50ish fee to list, refunded if you don't make it, and taxes if you go somewhere internationally (usually less than $100). I have had decent luck getting business class going to Europe.
Honestly, the Airman Trainee program is such little work to get the same travel benefits you would get as a pilot (apart from Jumpseat privileges).