Mutually Exclusive tags are convenient when it only makes sense to have one tag in a group applied at once.
It can be very helpful to group related tags together. For some groups, it only makes sense to use one tag at a time. Select a tag group, then check “Mutually Exclusive” in the inspector, and OmniFocus will ensure that only one tag in this group is added to an item simultaneously.
For example, let’s say you have a mutually exclusive group of tags called “Communications” that contains “Email”, “Phone”, and “Message”, and you have an action to “Call John about hiking on Saturday” tagged with “Phone”. If you add an “Email” tag to this action, OmniFocus will automatically remove the “Phone” tag.