During the mapping operation, you assign the external volume to an external volume group (ExG). This allows you to organize external volumes used for similar purposes to a particular group or groups.
For example, you might want to assign mapped volumes in the same external system to a specific ExG. Or, you might assign volumes used in a particular function, such as ShadowImage or Universal Replicator, to an ExG, even if the data is stored in different external storage systems.
You could also use ExGs to correspond to the external system's physical disk grouping, such as a RAID group.
You assign external volume group numbers during the mapping procedure.