Subfield codes identify the individual data elements within the field, and precede the data elements they identify. Each data fields contains at least one subfield code. The subfield code consists of a delimiter [1F16, 8-bit] followed by a data element id

Subfield codes identify the individual data elements within the field, and precede the data elements they identify. Each data fields contains at least one subfield code. The subfield code consists of a delimiter [1F₁₆, 8-bit] followed by a data element identifier. Data element identifiers defined in USMARC may be any ASCII lowercase alphabetic or numeric character. In general, numeric identifiers are defined for data used to process the field, or coded data needed to interpret the field. Alphabetic identifiers are defined for the separate elements which constitute the data content of the field.