4.2.5. Fields may be basic or compound.
4.2.6. The constituent parts of compound fields are called subfields, and are defined by the presence of keywords, delimited by the a start-of-field character.
4.2.7. Basic fields are fields which do not contain subfields.
4.2.8. The basic or compound fields composing the first level of definition of a message are called its primary fields.
4.2.9. All lower level constituents are by definition subfields, which in turn, may be basic or compound.
