If you are using direct addressing with keys, you can reserve space for future format-F records by writing a dummy record. To reserve or truncate a track for format-U, format-V, or format-VS records, write a capacity record.

Format-F records are written sequentially as they are presented. When a track is filled, the system automatically writes the capacity record and advances to the next track.

Rule: Direct data sets whose records are to be identified by relative track address must be limited in size to no more than 65 536 tracks for the entire data set.