It has been the droll observation of several that in writing Fuzzy Nation, what I’ve really done is write H. Beam Piper fan fiction.
My thought on that: Well, it is fiction, I am a fan, and I did write for my own amusement in a universe for which I didn’t have clearance in at the time — so, overall, yep, looks like fanfic to me, too, or at least some species of it, as I understand fanfic to be. Whether my subsequently getting clearance for the work makes it something other/different than fanfic is something I leave to others with a better knowledge base on the subject, but I suspect it just makes it fanfic authorized after the fact.
So, to the extent that people are saying “Hey! You wrote fanfic!” my response is “I suppose I did. And I had fun doing it.”