From SRB

A collection is a logical name given to a set of data objects. All data objects stored in SRB/MCAT are stored in some collection. A collection can have sub-collections, and hence provides a hierarchical structure. As a simple analogy, a collection in SRB/MCAT can be equated to a directory in a Unix file system. But unlike a file system, a collection is not limited to a single device (or partition). A collection is logical but the data objects grouped under a collection can be stored in heterogeneous storage devices. There is one obvious restriction, the name given to a data object in a collection or sub-collection should be unique in that collection.