MRNB - Background, Purpose, and Explanation

Hi, I support MM and PP for my company and I am wondering if someone can give me some good information about the MRNB transaction. I am looking for a business explanation of the transaction, it's purpose, and its usage. I have a user that says that they need to use the transaction - but I am unfamiliar with it and would like to provide some direction.

Any help would be appreciated!

Thanks in advance!