Use attributes

Private — meaning the volume can only be allocated when its volume serial number is explicitly or implicitly specified.
Public — meaning the volume is eligible for allocation to a temporary data set, provided the request is not for a specific volume and PRIVATE has not been specified on the VOLUME parameter of the DD statement.
Both tape and direct access volumes are given the public use attribute.

A public volume may also be allocated when its volume serial number is specified on the request.
Storage — meaning the volume is eligible for allocation to both temporary and non-temporary data sets, when no specific volume is requested and PRIVATE is not specified. Storage volumes usually contain non-temporary data sets, but temporary data sets that cannot be assigned to public volumes are also assigned to storage volumes.