This sets the maximum time (in seconds) the scanner will dwell on an inactive system. All channels in a conventional system will be scanned at least one time, regardless of the setting of this parameter.
For active systems:
After a transmission ends, the scanner applies the appropriate scan delay (0-5 seconds). Then, it checks to see if the hold time has expired. If so, it moves to the next system. If not, it continues to scan the current system until either the hold time has expired or a channel goes active.
The purpose is to keep the scanner from "hanging" on an active system and to give the user more control over the process. Set the time to 255, and it won't move off of the system for 4 1/4 minutes. Set it to 0, and it will stay on the system only briefly (one complete scan of a conventional system or a minimal dwell time on a trunked system).