They provide a consistent shorthand notation, telling a computer how to deal with the cataloging and making it easier for others to use it. Additionally, tags and subfields make possible computer-based systems that provide more access and flexibility than card catalogs. Without MARC, these services could not have developed and flourished as they have, nor could users deal consistently with records from the services. Libraries can add information to MARC records, define fields for local use and rearrange existing information without retyping catalog cards and risking transcription error. MARC provides the vocabulary and structure to produce catalog cards and online catalog access from a single set of information.
MARC increases choices for access and display while retaining the economics of shared cataloging.
Each MARC record must begin with a leader 24 characters long. If an electronic record does not contain a leader, it is essentially not a MARC record. Although the leader may not be required for the library's local system, it is important in a union catalog. Using the codes found in these two leader positions, the computer is able to determine the format of the record. The most common combination of codes in leader positions 5-7 is nam.