Physical chemistry of electrolyte solutions : modern aspects / J.M.G. Barthel, H. Krienk, W. Kunze ; edited by H. Baumgärtel, E.U. Franck, W. Grünbein ; on behalf of Deutsche Bunsen-Gesellschaft für Physikalische Chemie. (OCLC #39276859)

Similar to classifying a resource with a call number, you can specify a subject category using a code from one of many pre-defined lists.

That code is recorded in MARC 072 ǂa:

072 _7 ǂa QD ǂ2 lcco

If the category list is anything other than NAL (U.S. National Agricultural Library/AGRICOLA Subject Category Codes, which receive second indicator 0), the field has second indicator 7 and the category list is specified in ǂ7. This record has the code from LC classification outline, so it’s unsurprising that the code is also the first part of the LC call number:

050 00 ǂa QD561 ǂb .B29 1998