Subfield codes:
Subfield codes are two characters that precede each data element within a variable data field which require separate manipulation. A subfield code consists of a delimiter character followed by a data element identifier. A data element identifier may be a lowercase alphabetic or numeric character.
Subfield codes are defined independently for each field; however, parallel meanings are preserved whenever possible. 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 classification scheme itself.
For more information, select: