The topic of controlling scan timing is discussed in detail in the Scan Times section.
Please note: PRESCAN is retained mainly for backwards compatibility and should not normally be used. However it may be useful for a few situations with modern schedules. For example, if the user using DWELL wishes to push the scan further away from the preceding scan, then adding a positive PRESCAN, and lengthening the DWELL time by the same amount will do the job. That is an alternative to increasing TSETTLE in the station catalog, which the user may not normally want to touch.
Originally, PRESCAN was the mechanism for introducing gaps between scans. However that function is now better handled using the separate parameters GAP, PRESTART, and MINPAUSE.
PRESCAN specifies a time by which to offset the nominal start of a scan from what has been determined based on all other criteria. It is applied after all scan timing has been settled. It won't back up (negative value) over the stop of a previous scan, but it would be capable of messing up scan timing set by other parameters. It should only be used with considerable care as it should not be considered a carefully maintained feature.
Note that PRESCAN is treated as a UT time regardless of whether or not LST scheduling is specified (it is added to the start time after the schedule times are converted to UT internally in SCHED).
If you are writing VEX files, please see the discussion of restrictions on tape stop time given in the section on parameter GAP..