6.2. Auxiliary Terms
6.2.1. In order to provide a readable definition of fields, it is often useful to introduce auxiliary terms in the grammar description.
6.2.2. Auxiliary terms do not introduce a field or subfield and hence, are not associated with a particular keyword. However, they may appear in the definition of more than one field, or subfield, or auxiliary. For instance an auxiliary term like "date" may be used in the definition of many fields.
