Idle
Decides what a transporter does when it has nothing to carry.
This looks like a convenience and is not. A stopped transporter goes on holding the zones it stands on, so where a fleet idles decides how much of the guide path is unavailable to everyone else -- and an idle transporter standing where others need to pass will block them indefinitely, with no error and a run that completes looking entirely reasonable. Designing that out is what home spurs and staging areas are for, and this rule is how a model expresses the choice.
A rule is consulted only when nothing is waiting for the transporter. It cannot override the pool handing a transporter straight to the next entity in the queue, which would strand it while work waited.