When people say those things, they are referring to coach class in general. While "Y" is a fare code like B, M, H, Q, V, L, T and K, it's also used as a generic standin for "coach."

Note that "F" is often used for first class in the same way, even though "P" is full-fare first (although you'll see "FC" more often). And on NW, "J" is usually used to refer to WBC (although you'll see "WBC" more often).