Does the remote car door keyfob in today's automobiles change its code with every use?
With a rolling code system, both the transmitter and receiver calculate a code based on stored information that is not transmitted. Once the transmitter and receiver are "synchronized," a new code can be calculated in a sequence such that both calculate the same code as the next valid code in the sequence. In order for more than one transmitter to work, there is a system to allow the receiver to deal with more than one valid sequence. There is also a system to account for a transmitter skipping codes due to transmitting codes when out of range.