The BCS is a VSAM key-sequenced data set and contains volume, data set security, ownership, and association information for VSAM and non-VSAM data sets.

A BCS can also point to a volume on which only non-VSAM data sets and generation data sets reside. If the volume is SMS-managed, the VVDS also contains information about the non-VSAM and generation data sets. If the volume is not SMS-managed, the information is only maintained in the BCS.

Related information in the BCS is grouped into logical, variable-length, spanned records related by key. The BCS uses keys that are the data set names (plus 1 character for extensions). A control interval can contain multiple BCS records. To reduce the number of I/Os necessary for catalog processing, logically related data is consolidated in the BCS.

The cell is the smallest block of information in the BCS and might contain the name, volume, owner, and association information for a catalog entry.

Entries for SMS-managed data sets contain an SMS subcell. This subcell contains the names for the storage, data, and management classes for the data set.

VSAM components must have the same SMS attributes as their associated base cluster. These components must also be cataloged in the same catalog as the base cluster, and this catalog must be the one SMS defines as the default catalog. (The default catalog is the catalog the system chooses using only the catalog aliases and the data set names, that is, the catalog chosen when you let the system choose the catalog, instead of directing the search yourself.)

Temporary SMS-managed VSAM data sets have VVRs in the VVDS, but they do not have BCS entries.