Enhanced Storage uses virtual disks to manage physical disks and their associated data. In Enhanced Storage, a virtual disk is called a logical volume or a metadevice.
A metadevice is functionally identical to a physical disk in the view of an application; both a logical volume and a physical volume can host a file system and applications interact identically with both.
If, for example, you want to create more storage capacity, you could use Enhanced Storage to make the system treat a collection of many small slices as one larger slice or device. After you have created a large metadevice from these slices, you can immediately begin using it just as any "real" slice or device.
If you want to provide more redundancy to help protect your data against physical disk failure, you could use Enhanced Storage to create a and volume. These volumes duplicate data so that the failure of a disk does not cause the loss of data or an interruption in service.