You are right about "most favo(u)red nation", but see how it applies to music:
http://hubpages.com/entertainment/MFN-What-All-In-Means-for-...
MFN: What “All In” Means for a Music Supervisor
MFN is a key concept for music supervision
MFN Explained

A most favored nation clause, MFN, is a defined as “a commercial treaty in which the signatories agree to accord each other the same favorable terms that are offered in agreements with any other nation.”

How does this relate to music licensing?

A music supervisor will come across this basic concept countless times in their career and understanding it in its entirety is crucial.

MFN, as it relates to music licensing, simply means that if a party attaches this clause to their quote, they are due the highest negotiated fee as either the other parties involved in the song (master holder, publisher, co-publisher, etc.), or sometimes rest of the songs used in the production.

Let’s say you want to license a song called “How Do You Do?” for a minor spot in a commercial. I get the quote back from the publisher and they quote the song at $500 or MFN as to the master holder. Minutes later I get a quote back from the master holder with a $1,000 fee attached for the specified use of the song. Because of the MFN clause attached in the publisher’s quote, I am responsible for paying them the same $1,000 fee as the master holder, making my “all in” for this song $2,000.

If there are multiple publishers for the song and each publisher attaches an MFN clause to their quote, I am responsible for paying each person the highest negotiated fee.

An MFN clause can also be applied to other tracks within a project. If we were using “How Do You Do?” for a movie and they insisted upon an MFN clause as to the rest of the songs in the project, the music supervisor is responsible for paying the owners of “How Do You Do?” the highest fee of any other song used in the movie.

Licensing Music For Films, TV, Commercials, And Computer ...
http://dottedmusic.com/2013/legal/licensing-music-part-2/ Sep 17, 2013 - What is the amount of the Synchronization License Fee paid by the producer ... a “most favored nations” (also known as MFN) clause, which says that if ... be willing to agree to a lower fee than the publisher is willing to accept.