Yup, it's under the fourth paragraph under rules. Very much a "Should they be" like if they fit the role. Although if you are asking if it's a case of should the Masters be OCs or should they be Crossovers, then it doesn't really matter. For Masters, both Crossovers and OCs are both welcome. Canons are equally welcome too, such as Rin or Emiya. Being less particular on them than I am on Servants. They can be pretty much whatever, I just wanted it to be clear that they could be Crossovers as well if wanted.