The RESERVE macro serializes access to a resource (a data set on a shared DASD volume) by obtaining control of the volume on which the resource resides to prevent jobs on other systems from using any data set on the entire volume. There is an ENQ associated with the volume RESERVE; this is required because a volume is reserved by the z/OS® image and not the requester's unit of work. As such, the ENQ is used to serialize across the requesters from the same system.