Variable fields:
The data in a USMARC classification record is organized into variable fields, each identified by a three-character numeric tag that is stored in the Directory entry for the field . 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 (ASCII 1D16).
Variable refers to the fact that a field may vary in length and content, depending on the field and the USMARC format. There are two types of variable fields: variable control fields (00X fields) and variable data fields (0XX-9XX fields).
For more information, see: