Subfield codes:

Two characters that distinguish the data elements within a field which require separate manipulation. A subfield code consists of a delimiter (ASCII 1F16), represented as a $, followed by a data element identifiers may be an ASCII lowercase alphabetic or numeric character. Subfield codes are defined for purposes of identification, not arrangement. The order of subfields is generally specified by standards for the data content, such as the subject heading system/thesaurus rules.