OPELPRIO is used to specify two elevation ranges (4 values) to give high priority when optimizing a schedule. With OPTMODE=SCANS, scans on a source that falls within these elevation ranges will be taken every time while scans falling outside these ranges at all stations may be skipped depending on the setting of OPSKIP and on what happened on previous opportunities to observe the source. For geodesy projects, for example, priority might be given to very low elevations (for atmospheric determination) and very high elevations (to get good leverage on the parameter fits).
Note that the optimizing modes are somewhat experimental. If they are used, the output schedule should be checked carefully.