There are a certain number of pax per route who will travel in C or F. Your staff guesses at this under the Route Planning screen. Under ideal circumstances, they estimate this number of pax in each class will travel. They do not change into Y passengers; they just don't travel.
The type of seat affects the number of potential pax who will fly on your airline. If you are the only one flying the route, even if you have crappy seats, you will get a lot of those passengers. If you had excellent seats, you would get more, but you would be able to seat fewer of them.
Generally, seat type only comes into play in a competitive situation. In that case, the system looks at your CI, your RI, your seat type, the time of your flights, your frequency (huge), and the time it takes to fly between A and B for each of you. It then transmogrifies that into individual results.
Will seat type make a difference in a competitive situation? Yes.
Will seat type make a difference in a non-competitive situation? Not so much.