Apologies, I looked back on it and it's for guest posters.

1. A non registered user create a new thread.
2. A mod edit this posting. --> "edited by mod" is shown
3. A mod changed the category --> "edited by non registered user" is shown
4. A mod changed the category AND edit the posting --> "edited by non registered user" is shown

*edited to say it may be when it's moved category for a second time.