The 007 field contains information about the physical characteristics of an item in a coded form. The information may represent the whole item or parts of an item such as accompanying material. The 007 is mandatory for full-level records for microfilm and computer files. It is recommended for the other formats and levels of cataloging.
For the most part Houghton will be adding an 007 field to holdings records for "service copies", i.e. positive microfilm for the use of readers. A typical positive service copy would be coded: |a hd|afb---baca
Master negatives require slightly different coding, e.g.: |a hd|bmb---baca
(but see the MARC field documentation for precise codes). The third byte contains either a blank or a fill character.