Variable control fields:
The variable control fields have tags with the pattern 00X, where X may be any digit except zero (0), for example, 001, 005, 007. These fields are identified by a field tag in the Directory but they contain neither indicator positions nor subfield codes. The variable control fields are structurally different from the variable data fields. They may contain either a single data element or a series of fixed-length data elements identified by relative character position.
See also: