A new model revision is created when the Create model action is called on a document, a document revision, a model or a model revision. The model gets locked and an [in development] revision of the model is created. Unlocking a model creates a [current] revision of the [in development] revision.
Refer to the section Revision Status and the underlying subsections for more information on revision statuses such as [in development], [current], [accepted], and [published].