After you submit a video for monetization, YouTube may ask you for additional documentation demonstrating how you own commercial use rights to all elements in your video. This includes (but is not limited to) music, photographs, movie and video game footage plus more.
The written permission should either be a contract between you and the rights owner, or a letter from the rights owner stating that you are permitted to use their content commercially. Please note that messages, comments, and postings from social media websites (e.g., Facebook, Twitter, Google+, etc.), blogs or forums may not be sufficient to establish your commercial use rights. Elements you may want to include in this documentation include:
Explicit permission to use the rights holder’s content commercially.
Limitations/conditions concerning your use of content specified by rights holder, if applicable. Documentation must be valid worldwide for at least a year from the time of submission.
The URL of your video or, if you use the content in multiple videos, your channel name.
Electronic signature with date (this can be as simple as the rights holder writing out his full name at the bottom of the document).
The above material is being provided solely for educational purposes and is not legal advice. You should only seek legal advice from a lawyer or legal representative.