An anchor is a document structure that is more virtual in nature and can combine other structural units into a group. Such anchors are, for example, the structures Multi-volume Work or Journal, which holds all volumes of the work or all volumes of the journal together. The anchor itself has no content files - i.e. no image set with image files, it consists only of a structural unit that has descriptive metadata. The essential metadata is an identifier with which the anchor can be clearly identified.