Unable to digup anything on the web or sift thru the forums here, figured I'd ask here.

What exactly is a Premium Econ Sell Up Fare?

I'm looking at an itinerary that has a fare from BOS-DFW-EZE, and the fare basis shows NLN0NSP1
BOSDFW books into Y
DFWEZE books into P

I'm fairly new to all the backend fare stuff but my general understanding is that the fare basis shows it's underlying fare bucket is N, despite what it books into Y/P mentioned above? How does that work in general? I also tried searching for the fare basis in Expertflyer but it didn't return anything. Also relatively new to EF, but I thought the fare basis can be searched for since it seems like a public fare than I can search for on ita and book on AA.com

Any experts care to educate a novice on this topic as it's gotten me intrigued

Sample itinerary from matrix below
Matrix - ITA Software by Google