14.8.8 NOSPAN Clause
  By default, sequential files allow records to cross or span block
  boundaries. If records cross block boundaries, RMS packs
  records into the file end-to-end throughout the file, leaving
  space for control information and padding.

  The NOSPAN clause overrides this default, forcing records
  to fit into individual blocks (with space provided for control
  information and padding). When block boundaries restrict
  records, fixed-length records must be less than 512 bytes,
  and variable-length records less than 510 bytes. This can
  waste extra bytes at the end of each block. However, when
  records span block boundaries, RMS writes records end-to-
  end without regard for block boundaries. For example, if you
  specify NOSPAN, only four 120-byte records fit into a disk
  block. If you do not specify NOSPAN, BASIC begins writ-
  ing the fifth record in the block, and continues writing that
  record in the next block. This minimizes wasted disk space
  and improves the file's capacity, at the minimal expense of
  increased processing overhead.