AACR2 defines supplement as:

An item, usually issued separately, that complements one already published by bringing up-to-date or otherwise continuing the original or by containing a special feature not included in the original. The supplement has a formal relationship with the original as expressed by common authorship, a common title or subtitle, and/or a stated intention to continue or supplement the original. See also Module 17. (AACR2 12.7B7j).

When a supplement is cataloged separately from its parent serial, link from the supplement to the parent serial, and from the parent record to the supplement. If there is more than one supplement, reciprocal links may be added to all related records. If there are many supplemented, it may be impractical to list them all on the parent entry. (6) When cataloging the supplement, give an added entry for the parent serial, unless the supplement has a common title that is identical to the title proper of the related serial (LCRI 21.28B). Example:

See also:

14.3. Horizontal relationships