Somewhat unique to OpenStack Nova is the ability to shelve an instance. Instance shelving allows you to stop an instance without having it consume resources. A shelved instance will be retained as a bootable instance, as well as its resources assigned such as IP address, for a configurable amount of time, then deleted. This is useful as part of an instance life cycle process or to conserve resources.
Note
Stopping versus shelving?
Stopping an instance does not free up the amount of resources still available as part of your quota as the assumption is that you will be starting that instance back up again after a short period of time. You would not be able to start a stopped instance if you didn't have any free CPUs or GBs of RAM left of your assigned quota. A stopped instance's resources are still considered used resources to the OpenStack Compute scheduler.
Shelving, however, frees up these resources, but still allows you, at a later date, to access the shelved instance...