A
museum is distinguished by a
collection of often unique objects that forms the core of its activities for ,
education,
research, etc. This differentiates it from an
archive or
library, where the contents may be more paper-based, replaceable and less exhibition oriented, or a
private collection of art formed by an individual, family or institution that may grant no public access. A museum normally has a collecting policy for new acquisitions, so only objects in certain categories and of a certain quality are accepted into the collection. The process by which an object is formally included in the collection is called
accessioning and each object is given a unique
accession number.