Simply put, the VM is R/W using a snapshot. Changes are kept on a datastore within the VMware environement, while the temporary datastore created from the NBU storage is read-only. One you storage vMotion to a datastore, and close out the restore attempt, the change file is written back to the VM.
It's fully functional, but meant for to be a short-term solution to get the box up and running, until you can get it to production storage.
