Yes, you can assign a base layer during centralization. Base layers are used by IT to keep devices in sync with apps and patches.
I think you mean assign vs. provision as a provisioning of a base layer will wipe the end point clean and you do not have to have the user data necessarily backed up (centralized). An assign of a base layer is done after a CVD has been centralized and the user data is now able to be applied back to the base layer when completed. It also will do the base layer operations on the server side in the data center and then be pushed out to the end point.
You can centralize any machine by installing the Mirage client, then "centralize" it. You do not have to apply a base image to it once it is in Mirage. However, you won't be able to deploy app layers unless you first assign it a base layer.
Base layer also allows you to make OS updates without having to use another process (WSUS, SCCM).
You would have a reference machine running, install the updates on it, update the base image, then push it out to the CVD's. (but you need a base image assigned).