I recall hearing once that there was a difference (due to guild rules) between the use of "and" and "&" when discussing collaborations. For example, a film that has
Screenplay by Bob Loblaw and Dr. Dre
means that the writing was divided differently than
Screenplay by Abraham Lincoln & Bobcat Goldthwait
which is different again from one that says
Screenplay by Alan Smithee and Trogdar the Burninator & Guy Ritchie
What are the rules, and is it consistent amongst the guilds? I.e, do Director collaborations also have the same semantics?