The creative brief (assignment brief)
The creative brief tells the creators what message you want to convey and to whom. It also lays out all the rules, regulations, brand assets and everything else you want your videographers to know before they get started. It's important to be specific, but still provide some latitude for creators to think outside the box. Do keep in mind that not everyone speaks your unique brand marketing language, so you'll need to make sure the brief is easy for outsiders to understand.

Brand guidelines
As a brand marketer who is very close to your product, you know what is acceptable and what will never make it through the legal department. Be sure to let the creators know what the "no-no's" are. For example, are there logos that cannot be altered? A brand slogan that has to be said a certain way? Certain actions to avoid like violence or sexual references? You want to give your videographers creative freedom, but by setting some basic guidelines, you ensure that videographers understand the constraints within which they can create. This will lead to better, more usable submissions.