Electronic holdings records, in general, are composed of two interlocking standards:
- Display standard, which determines the conceptual data elements and how they display to staff and patrons.
- Communications standard, which transmits the holdings data to unrelated computer systems. It may also store the holdings record internally in a particular system.
The relationship between the two types of standards can be thought of as a "box within a box":
The display layer is officially referred to as the content designation, and the communications layer is officially referred to as the record structure.
Within integrated library systems (ILSs) in North America, the current holdings display standard is Z39.71, and the official communications standard is MARC 21 for Holdings Data (MFHD). The North American landscape can be illustrated as follows: