Adding a segment manager to a disk allows the disk to be subdivided into smaller storage objects called disk segments. The add command causes a segment manager to create appropriate metadata and expose freespace that the segment manager finds on the disk. You need to add segment managers when you have a new disk or when you are switching from one partitioning scheme to another.
EVMS displays disk segments as the following types:
Data: a set of contiguous sectors that has been allocated from a disk and can be used to construct a volume or object.
Freespace: a set of contiguous sectors that are unallocated or not in use. Freespace can be used to create a segment.
Metadata: a set of contiguous sectors that contain information needed by the segment manager.