Variable fields:

The data in a MARC record is organized into variable fields. Variable refers to the fact that a field may vary in length and content, depending on the field and the MARC format. Each field is identified by a three-character numeric tag which is stored in the Directory . Each field ends with a field terminator character. The last variable field in a record ends with both a field terminator and a record terminator character (ASCII 1D hex).

There are two types of variable fields: the variable control fields (00X fields) and the variable data fields (0XX - 8XX fields).