SCHED will not normally allow schedules to be made that use untested VLBA or MarkIV recording modes. The tested modes are listed in the document OK_modes.vlba accessible from the VLBA WWW home page. This is to try to prevent the situation where the correlator group is forced to work on some modes that are not otherwise important at the moment just because some user decided, probably in ignorance, to use one that doesn't work. This test is done only for the VLBA correlator. Essentially all modes that are possible have been approved as of 2001 so this should not be needed unless (as was the case for a long time) the tables in SCHED are not up to date.
In order to test new modes, or in cases where the documentation is out of date, it is necessary to schedule untested modes. To bypass the test in SCHED, specify MODETEST in the setup file.