Where do you draw the line between the Tolkiens blessing second-party works as being the "official lore" vs leaving them on the level of self-published internet fan fiction? Of which there is absolutely no shortage?

Is it just a quality bar? Some sense of how much it "fits" with the existing world while still expanding on it in meaningful and plausible ways?

I get that Star Wars novels are a template here, but that's not exactly a flattering comparison. Other cases like Robert Jordan and Robert Ludlum seem also to be a bit mixed.