Formal USMTF Excerpt: Field Formats.A field format establishes and is derived from the structure of allowed field content, i.e., the words (literals) or codes used in completing a set.A field format may support either elemental concepts, such as the name of a location, or a combination of elemental concepts, such as date time group, that are linked by common usage or formatting requirements.The field format, therefore, is the fundamental building block in the overall structure of formatted messages and provides the bridge between that structure and the vocabulary of allowed literals and codes.The USMTF categorizes the vocabulary used in formatted messages by use of the Field Format Index Reference Number[FFIRN] and Field Use Designator Number[FUDN].The FFIRNs / FUDNs to be used in a particular set format are identified in Paragraph 5.3.1.The USMTF Catalog of Field Formats provides, by FFIRN / FUDN, the specific data items and corresponding data codes to be used as field content.A detailed discussion of rules concerning fields is provided in Paragraph 4.3.6 below.
Layman's Explanation: A Field is simply some data in a set governed by some rules. It can have an optional descriptor that identifies it as well as the data itself, to which i call content.
The rules are up to the Set and enforced by the FIRN/FUDN applied. Programmatically we use FFIRN_Validators for parsing and enforcement.