The close routine handles end-of-data-set processing on output tapes. When a write operation is the last operation that occurs before closing a data set (for OUTPUT, OUTIN, OUTINX, EXTEND or INOUT) or when no output is written before closing (for OUTPUT, OUTIN, OUTINX or EXTEND), the close routine writes two tape marks. When you are writing on a cartridge (not a reel), the system can perform block count checking. See Verifying block count.