OPMINSEP is the parameter for OPTMODE=HAS that is used to prevent two scans on the same source from being scheduled too close together. SCHED determines the default spacing of scans -- evenly spaced across the available time -- regardless of whether the user overrides that default by specifying the desired hour angles. That default spacing is multiplied by OPMINSEP to get the minimum separation. Once a scan is scheduled, another scan on that source will not be scheduled until after that minimum separation has passed. A different value can be given for each scan.
See the discussion of the possible optimization modes in the section on OPTMODE for more information on OPTMODE=HAS