Directory:
The Directory is an index to the location of data within a record, containing information on the tag, field length, and starting location of each variable field within the record. Each entry is 12 character positions in length. Directory entries for variable control fields appear first, sequenced by tag in increasing numerical order. Entries for variable data fields follow, arranged in ascending order according to the first character of the tag. The stored sequence of the variable data fields in a record does not necessarily correspond to the order of the corresponding Directory entries. Duplicate tags are distinguished only by the location of the respective fields within the record. The Directory ends with a field terminator character.
Users typically do not have access to the Directory; it is generated automatically by the system. MARC Bibliographic, Authority, and Holdings format records all have identical directory structures.
For more information, see: