Title-manifestation, which by definition has no change in content, does not work for serials, since change in content occurs with every new issue as a matter of course. A sixth entity is therefore defined for serials, since they continue over time not by replacing themselves with revised editions, as monographs do, but rather by a continuous stream of issues each of which is a new compilation of other works (articles). Change in title has a profound effect on users' ability to find the serial work that contains the article work they are actually seeking. This new entity is called a serial title. In effect, it is used to summarize all manifestation changes (changes in publication information) that occur during the time that a serial's issues all bear the same title before the next major title change. Since a serial's content is continuously changing, it is possible for the subject matter of a serial to change below the expression level, and for subject headings to be serial title-change specific. This would not be possible for a monograph below the expression level, since any content change would create a new expression.
Serials that exist in several simultaneously released editions, such as different language editions or editions with different content for different markets, on the other hand, do seem to fit into the definition of expression that works for monographs as well.
According to the FRBR model, every item contains a manifestation, an expression and a work. Since title-manifestation is a kind of manifestation and since serial title plays somewhat the same role as title-manifestation for monographs, the following permutations are all possible according to these rules:
item — manifestation — expression — work (monographs and integrating resources)
item — manifestation — title — manifestation — expression — work (monographs)
item — manifestation — serial title — expression — work (serials)