An allocation class is a number from 1 to 255 that serves as part of a device name to provide unique and unchanging names in the form $allocation-class$device-name to uniquely identify shared devices, files, and data. Allocation classes are required in OpenVMS Cluster configurations where storage devices are accessible through multiple paths. Without the use of allocation classes, device names that relied on node names would change as access paths to the devices change.
Prior to OpenVMS Version 7.2, all nodes with direct access to the same multipathed device were required to use the same nonzero value for the node allocation class. OpenVMS Version 7.2 introduced the MSCP_SERVE_ALL system parameter, which can be set to serve all disks or to exclude those whose node allocation class differs.
Port Allocation Class
OpenVMS Version 7.1 introduced a port allocation class, which is specific to a single interconnect and is assigned to all devices attached to that interconnect. Port allocation classes were originally designed for naming SCSI devices. Their use has been expanded to include additional devices types: floppy disks, PCI RAID controller disks, and IDE disks. The use of port allocation classes is optional.