You can attach a volume to a cloud server at the volume creation time. Alternatively, you can do it later from a "Volume details" page in the Customer Portal. Volume can only be attached to a single cloud server instance at a given time, but also can be detached from one instance and attached to another.
At the "Volume details" page you can see the Cloud Server to which a volume is attached to, and the corresponding device name:
In the console of a cloud server you can use "lsblk" command to list all available block devices:
$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT vda 253:0 0 2G 0 disk └─vda1 253:1 0 2G 0 part / vdb 253:16 0 1G 0 disk