In order to enable versioning The process of tracking changes that you make to your model file. in a file, select Model|Versioning... from the main menu. The first time you do this, a message is displayed asking you to confirm that you want to enable versioning.
After confirming your request, you will immediately be prompted to create a version of the file (take a "snapshot" of the current model file and "stamp" it as a version).
Once you create the first version, versioning will be enabled, and will remain enabled until you choose to disable it.