KEY IS phrase

Fields named in KEY IS phrases of SELECT statements are included as fields in the XFD. Other fields that occupy the same areas as the key fields (by either explicit or implicit redefinition) are not included by name, but are mapped to the key field column names by the data dictionary.

Remember, if the field named in the KEY IS phrase is a group item, it will not become a named field in the XFD unless a USE GROUP directive is used (see "Understanding how the XFD file is formed").