whats the difference between the 737-800 and the 737-900? They both carry max 189 pax in high density seating and the 738 has better range depending on the range.
The 737-900 has more floor space than the -800 so you can theoretically put more premium seats in. The 189 pax limit is an artificial limit (in real life, not just AWS) due to the number of exit doors. The 737-900ER added another pair of doors to increase the certified capacity, but it has the same (or nearly the same) floor space as the non-ER 739.