A foreign volume is a volume mounted on the system but not processed in terms of OpenVMS default file systems (Files-11 for disks and ANSI for magnetic tapes). Usually, these are volumes initialized in a different file systems, but Files-11 and ANSI volumes may also be mounted /FOREIGN for reasons such as backups.
If you mount a volume with the /FOREIGN qualifier, the program you use to read the volume must be able to process the labels on the volume, if any. The OpenVMS operating system does not provide an ancillary control process (ACP) to process the volume.
You must mount DOS-1 and RT-11 volumes with the /FOREIGN qualifier and process them with the Exchange utility.
The default protection applied to foreign volumes is RWLP (Read, Write, Logical I/O, Physical I/O) for the system and owner and no access for the group and world. If you also specify /GROUP, group members are also given RWLP access. If you specify /SYSTEM or /SHARE, the group and world are both given RWLP access. Note that the /GROUP, /SYSTEM, and /SHARE qualifiers do not alter the default protection.