is
Whether this transporter could be sent somewhere if it were asked right now.
Distinct from being unallocated, and the distinction is the point. A transporter is unallocated when nobody holds it, which is a fact about ownership. It is dispatchable when nobody holds it and it is able to move, which is a fact about the vehicle. The two differ for a transporter that is repositioning under an idle disposition rule and is halted by its movement gate part way, or one that something else is towing: it belongs to nobody, and it is going nowhere.
A pool sends only dispatchable transporters, and an allocation rule is offered only these, so a rule never has to know this property exists.