Anyone can save changes to the diagram, even if they are not actively editing the diagram at the time using 'Pause and Edit'. The reason for this is so that specific items such as Attributes, Operations, etc. can be pushed through to all users. Multiple people can also have 'Pause and Edit' mode activate at the same time and be making changes, so there's no limitation on how many users can be editing the diagram at the same time.
If only a single user should be editing the diagram at a time, it's suggested that a user lock is applied to the diagram in question, this will prevent other people from modifying the diagram or enabling 'Pause and Edit' mode.
Please note, It is recommended that a user locks the diagram before editing, when the diagram is a shared resource. It is a common practice for users to lock a shared resource before editing.
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