here's the deal ... you register the tag with SUNPASS as a backup to the transponder .....
When you go thru a toll gate the system will
1 - look for the transponder, if found it processes the toll, else
2 - the tag is read and first the SUNPASS database is checked to see if the tag is associated with a valid account, if found the toll is processed normally - this backs up the transponder in case of a boo boo INCLUDING the transponder not in the car {I only have mine with me about half the time, it moves around 5 cars, all 5 tags are in the system - associating multiple tags only works with the portable $20 version; but this is why registering your rental tag is recommended, it facilitates the backup 'toll by plate' to the SP account - and as noted removing a rental tag promptly is a good idea} else
3 - the 'fleet database' is checked, this is where most rental car companies have their data and the tolls are managed by a third party ... if found the toll is processed thru the third party who bills you thru the rental company; else
4 - if all this fails the system processes as 'not found' and what happens next depends on where in the state you are; this usually is a private vehicle so I'll keep it simple. In areas where the tolls are 'electronic only' {Miami area primarily}, the system collects your tolls for a month and uses the address associated with the TAG to send a bill with a small service charge. Failure to pay the bill is a violation. In areas with cash option for payment (greater Orlando, most of the turnpike north of Miami), a violation can be issued as you have simply 'run the toll booth' but folks have said a one time boo boo did not result in a violation ... they only go after repeat offenders.