I thought that N was just the most deeply discounted fare bucket. For example, United lists the upgrade priority by fare class as Y, B, M, E, U, H, Q, V, W, S, T, L, K, G, N, and this Wiki lists N as the lowest coach fare class.

However, I'm finding many fares on line where the expert mode reports G0 N9, or even sometimes T0 L0 K0 G0 N9. So is it the case that N is cheaper but more available, only I can't book it because it carries additional fare restrictions? Or is N only available for bulk ticketing or employee discount tickets or some other special purchasing channel?