19.1.1 Allocating and Mounting a Tape
  You should allocate the tape unit to your process before
  starting file operations. For example:
  $ ALLOCATE MT1:

  This command assigns tape drive MT1: to your process. You
  must also set the tape density and label with the MOUNT
  command. Optionally, you can specify a logical name to
  assign to the device, in this case, TAPE.
  $ MOUNT/DENSITY=1600 MT1: VOL001 TAPE

  When mounting a TK50, you cannot specify a density.
If the records do not specify the size of the block (no value
  in HDR 2), specify the BLOCKSIZE as part of the MOUNT
  command. For example:
  $ MOUNT/DENSITY=1600/BLOCKSIZE=128 MT1: VOL020 TAPE

  Alternatively, you can use the $MOUNT system service to
  mount tapes.