A removable media library contains all the tape and optical volumes that are available for immediate use, including the shelves where they reside. A removable media library usually includes other libraries: system-managed libraries such as automated or manual tape libraries; and non-system-managed libraries, containing the volumes, shelves, and drives not in an automated or a manual tape library.

In the removable media library, you store your volumes in shelves, where each volume occupies a single shelf location. This shelf location is referred to as a rack number in the RMM TSO subcommands and in the DFSMSrmm ISPF dialog. A rack number matches the volume's external label. In DFSMSrmm volume serial numbers are used to identify volumes and to identify the volume label. DFSMSrmm allows you to define a volume using a different serial number than is recorded in the volume label. In this way you can define volumes with duplicate volume serial numbers. DFSMSrmm uses the external volume serial number to assign a rack number when adding a volume, unless you specify otherwise. The format of the volume serial and rack you define must be one to six alphanumeric, national, or special characters.