Positive responses must include a minimum of 3 controls otherwise just bid game force. The book says 4 but think the person can figure out slam with guarantee of 3.
Responder is transferring to a 5+ card suit. So accepting the transfer is confirming a fit and rejecting it is showing no 3 card support your own 5 card suit if you bid it at the lowest possible level. A minor should typically have 6 so can accept with 2.
Once transfer is excepted should bid controls starting at 0-3 and one step up for each
If rebid is 2NT instead of accepting the transfer systems are off (with the exception of Gerber) to allow showing additional length and find slam
If the transfer is TO 2NT then the systems are on
If transfer is rejected and the suit is rebid without an agreed suit it is showing only 2 of that suit.