The 007 field encodes information about the physical characteristics of an item. 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 records will feature a 007 field in holdings records for "service copies"; an example of this would be a positive microfilm intended for patron use. That typical positive service copy would be coded:
$$a hd|afb---baca
Master negatives require slightly different coding:
$$a hd|bmb---baca
See the MARC field documentation for precise codes.
The third byte contains either a blank or a fill character.