Preemptive Priority
Preemptive priority is a feature that works on Motorola systems. If the Motorola system has priority channels, and you assign one of those TGs as priority in the scanner, then when one of those channels becomes active, the scanner will jump to it even if it is on a different system voice channel (i.e. it preempts your current reception).
This feature relies on the system itself having talk groups assigned as priority (so the active channel information is included on voice channel low-speed data). If a talk group you assign as priority is not also a priority channel on the system, then the scanner will use standard trunking priority for the talk group.
Standard trunking priority is also used for other types of trunked systems. In this mode, the scanner will assign higher priority to talk groups that are active only when the scanner is evaluating the control channel. The scanner can't interrupt an ongoing transmission to check for trunked priority activity. Also, it can only check for priority traffic on the trunked system it is currently monitoring.