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When working in a repository you have the option to set a model version to read-only.
When Read only is selected, the model version will be in read-only mode and no changes can be made to it. If you hold the lock on this model version it will be dropped, and any open diagrams will become read-only.
While a model version is read-only you are not able to obtain a lock on it and diagrams of this model version or any groups or schemas in it will be opened in read-only mode.