Inside of each auto attendant, except auto attendant no. 1, is option 6, Line Assignment. Assign lines to AA 2, 3, or 4 as appropiate, line not assigned will default to AA 1.
Centrex Transfer does not use a different line for transfer, it flashes the line to receive 3 way calling dial tone from the telco, dials the destination number, and then disconnects. This requires "call transfer/disconnect" service from the telco on the lines.
Remote Call Forward uses extensions that are forwarded to outside numbers, the selector code transfers to a specific extension number that then uses the next available line in that extension's Auto Line Select to dial the outside number and then connect the calls together.