The 007 field, which is intended for machine manipulation, encodes information about the physical characteristics of an item. That 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 other formats and levels of cataloging.
For the most part, Houghton records will feature a 007 field in holdings records for so-called service copies; an example of this would be a positive microfilm intended for patron use. That typical positive service copy would be coded:
007 $$a hd|afb—baca
h: microform
d: microfilm reel
leave the third byte blank
a: positive
f: 35 mm
b: normal reduction
leave the seventh byte blank unless recording a reduction ratio
b: black and white (or monochrome)
a: silver halide
c: service copy
a: safety base, undetermined
Master negatives require slightly different coding:
007 $$a hd|bfb—baca
h: microform
d: microfilm reel
leave the third byte blank
b: negative
f: 35 mm
b: normal reduction
leave the seventh byte blank unless recording a reduction ratio
b: black and white (or monochrome)
a: silver halide
c: service copy
a: safety base, undetermined
See the MARC field documentation for precise codes.