The works described in a bibliographic catalogue may be related to each other, through various kind of relationships.
Works may be considered equivalent since they share the same intellectual or artistic content (eg. copies, facsimiles, reprints) even if the medium where the expression is recorded is different, such as photocopies and microfilms.
A work can be derivative of another work. In derivative works the content is modified and it depends on the extent of this modification to to determine if the outcome will be considered the same work with a new expression, or a new work. Slight modifications, such as a translation, result in new expressions of the same work while significant changes such as an adaptation or change of genre results in a new work.
A work may be descriptive of another work. It may describe, critique, evaluate or review other works, such as reviews, commentaries, annotated editions etc.