The concept of bibliographic level is introduced to allow the identification, within a single reference, all bibliographic entities of which the reference itself is composed and which are required to make the description of a documentary unit complete.
The bibliographic level are:
M: Monographic level. For example: A book
AM: An analytic of a Monograph. For example: Chapter of a book
MS: A Monographic Series. A complete documentary unit issued in a predetermined number of parts
AS: An analytic from a serial. For example: A paper from a Journal
AMS: An analytic from a Monographic Series
For more information see the cataloguing manual under: Bibliographic levels