CAT=Chicago Area Theatres
LORT=League of Resident Theatres
Each type of theatre is operated under a different contract. Usually the contract is based on the size of the house, but not necessarily.
Pay wages also differ depending on the contracts. The number of number of performances can also factor into what contract is required of each theatre as well as the number of Equity actors required to be contracted.
There are a lot of differences between the contracts and Equity will often allow a theatre to "bend" the rules a bit depending on the circumstances. But there is a rule book for each type of contract that covers everything you need to know before you sign.
PEACE.