Marvel Comics has been selling the rights to make films based on their material since the 1990s. Thus, the rights to involve certain Marvel characters in screenplays are restricted according to which studio owns them. The creation of Marvel Films in 1993 (now known as Marvel Studios) allowed for Marvel to do a reaquisition in some cases.
The issue seems complicated, but The Geek Twins simplified things with their graphic that explains which studio owns what. What crossovers can take place, allowing for who to fight whom?
Visit this page to see their graphic in three sizes, read a listing of studio rights, see The Geek Twins' references for their undertaking as well as to view other graphics they've created.
Via Design Taxi | Image: The Geek Twins