I was under the impression that in Motorola systems, the difference between message and transmission trunking is in how long a channel is held open after the end of a transmission. That with transmission trunking, every time a transmission terminates, the repeater is de-keyed and returned to the pool, while with message trunking, the repeater is held open for a defined period of time to allow a reply to occur on that same repeater, reducing the need for the replying unit to wait for channel assignment when the system is busy.