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
practice of inserting 007 in holdings record has changed after migrating from Aleph to Alma, in addition to Harvard Libraries moving away from the single record policy towards the multiple record policy. Consult the Metadata Standards Working Group's Best Practices on 33X and 007 in Alma Bibliographic and Holdings Records.
Basically, when the format described in the bibliographic record matches the format of the holdings record, do not copy 007 from bib to holdings. However, when the format represented by the holdings record is not reflected in the bibliographic record, create appropriate 007 field in the holdings record. See the MARC field documentation for precise codes.
N.B. There is no need to remove extraneous 33x or 007 fields from legacy holdings records, although it does no harm to do so.