Board Thread:Administrative/@comment-1713281-20190509171612/@comment-3406131-20190509175105

Void Indigo by Steve Gerber seems to be a good exercise to define the line:
 * It is a creator-owned characters (MGN and Epic)
 * It references Marvel topics (Sominus, Zhered-Na)
 * (side-note: those references were from Gerber's own work, and Gerber had copyright fights with Marvel, so that could be, as Nausiated put it "another one of his ways of 'taking back' his own work")

Other cases, such as the Haunt of Horror: Lovecraft Vol 1 or the early mutants is way more left to interpretation.