Thanks for the clarifications!

My original itinerary reads:
Cabin: United Economy
Seat: Seat: 05C
Fare Basis Code: SR10OTDN
Booking class: S

My itinerary after checking in reads:
Cabin: United Economy
Seat: Seat: 05C
Fare Basis Code: N/A
Booking class: Y

My boarding pass reads:
Y-Cabin

And that's why I'm confused. I didn't realize there is a difference between booking class and cabin. I thought they were the same thing.