Things are always a little different from state to state - this is what MA says:
Can I add a rental vehicle to my account?
If you are planning on using a rental vehicle you should add it to your account prior to using the rental vehicle. You can update your account on-line or call customer service. The rental vehicle should be removed from your account 48 hrs after you have returned the rental vehicle. It is recommended that you check with the rental agency about their policy for using your transponder in their vehicle.
May I use / move my transponder from one vehicle to another vehicle?
Yes, the transponder may be used in another vehicle provided both of the following conditions are met:
All vehicle license plate numbers in which you intend to use your transponder must be listed on your account.
All vehicles which you use your transponder must be of the same class. For example, if you registered your transponder with a 2-axle, 4 tire vehicle with non-commercial license plates, you may only use that transponder with other 2-axle, 4 tire vehicles with non-commercial license plates listed on your account.
NOTE: Commercial license plates, including dealer plates, are not permitted on individual accounts.