The less familiar kind of variable field is the variable control field. These fields, which are tagged 00X, contain neither indicator positions or subfield codes. Tag A tag is a 3-digit code that identifies a variable field. For example, a personal name entry is identified by tag 100, the title by tag 245, and a general note by tag 500. Indicators are associated with each tag in a MARC record and are used to further describe a particular field.
Indicators appear as numerals immediately following their corresponding tag.
In some fields numerals occur in both indicator positions, in other fields an indicator may occur in the first or the second position, and in some fields both indicator positions are blank. The age of innocence. Subfields are smaller logical units of information within each field. Each subfield is preceded by a subfield code which is a special character that identifies that certain information is to be found in a particular subfield. For instance, subfield c in the 260 field will always have only the publication date or copyright date or both. subfield c in the 260 will never carry any other information besides date information.