Ephemeral vs Persistent storage option Create New Volume on CREODIAS
Volumes created in the Volumes > Volumes section are persistent storage. They can be attached to a virtual machine and then reattached to a different one. They survive the removal of the virtual machine to which they are connected. You can also clone them, which is a simple way of creating a backup. However, if you copy them, you might also be interested in Volume snapshot inheritance and its consequences on CREODIAS.
If you follow the instructions in this article: VM created with option Create New Volume Yes on CREODIAS and set Delete Volume on Instance Delete to No, the boot drive of such virtual machine will also be persistent storage. You can, for example, use this feature to perform various tests and experiments.
If you do not need persistent storage, use ephemeral storage. It cannot be reattached to a different machine and will be removed if the machine is removed. See the article VM created with option Create New Volume No on CREODIAS on how to create a virtual machine with this type of storage.
You may find more information regarding this topic in the official OpenStack documentation on design storage concepts.