Each unit has 20 + 1 ids.
The 20 ids are basically loaner transponder ids (check the last digit).
MRT has an unknown number of transponders from AMB used for cloning ids. If you don't clone your AMB one, you get one of their numbers (in addition to the 20). There is a chance of running into someone else having a transponder from MRT with the same id, in which case you'll have to use one of the other 20 ids by pressing the button. You will never conflict with an AMB transponder.
Very happy with mine btw.
