A special key-sequenced data set (KSDS) with an index containing extensive data set and volume information that the Virtual Storage Access Method (VSAM) requires to locate data sets or files, allocate and deallocate storage space, verify the authorization of a programme or operator to gain access to a file, and accumulate usage statistics for data sets or files. VSAM catalogues have been functionally replaced by integrated catalogue facility (ICF) catalogs. VSAM catalogues are not supported by z/OS.