Each fixed field element displays as a mnemonic label followed by coded information. For example, the label Lang stands for Language, a fixed field element that identifies the language of the work. The code for English is Eng. The fixed field in MARC records is very important. Inclusion of coded information in the fixed field allows both utilities such OCLC and participating libraries to take advantage of a computer's capabilities for retrieval and manipulation of data.
Codes provide for economy of space in computer systems.
There are two kinds. The more familiar are the variable data fields. These are of varying length and carry any text or other data needed to describe an individual item such as the main entry heading, title, call number, etc. This is the information that is displayed to catalog users. Each variable field of this type is identified by a three digit code called a tag, followed by two character positions reserved for indicators.