NHEADPOS is the number of head positions to use at the station. The traditional number for Mark IIIa sites is 12. However, the VLBA uses a head position sequence with 14 positions. This allows more data to be put on a tape. This input is provided mainly to allow the number of head positions to be forced to 12 to match a schedule made to allow time for tape changes every 12 head positions. This is likely to be common for Mark III projects.
The MkIV schedules are currently scheduled with similar transverse tape format as the VLBA; normally the same 14 head positions are used. For recordings made with 2 heads, 6 head positions is the default. For S2 recorders this parameter is ignored, the different groups in an S2 format can be set by TPMODE.