When creating Events, I usually create them as Date-Range-Based. Occasionally, when reviewing the files the Event contains, I encounter some that (although are in fact dated within the Event's TimeWindow) have nothing to do with the Event. These Photos have been taken at another location, by someone not participating in the Event, so they don't belong as part of it.
IMatch of course gives us other Event Types to deal with that situation.
What I have been doing for this is to select all of the files in the Event, and then create a new Event (Ctrl+Alt+E) with a similar name (which becomes a File-Based Event). Then I delete the desired files in the new File-Based Event, delete the old Event, and rename the new one to the original name...(I have since learned that I can use the same name for the new event at its initial creation, so the rename is not necessary).
But if I have already done other config work in the original Event (setting Cover Files, Description, etc.) before noticing that the Event "needs to be converted" to a File-Based one, it can be just a little bit more work.
It would be nice if IMatch offered an option to "convert" an existing non-File-Based Event to a File-Based one (while retaining all of its other properties).