A PageSeeder concept that was redefined with the release of ver 5.96xx.
As a server-wide, unique identifier, the publication id makes it straightforward to find the root document in collections that use xrefs to embed or transclude content into a single hierarchy. While the publication id is typically attached to a references document type, any document can be a publication.
With the publication config, the same document can simultaneously exist in multiple publications while being numbered according to different schemes, positions in the publication hierarchy or collection-level versioning. See Publications and publication types.