You can switch between Economy/Business (for example) on each screen; so book the outbound flight(s) in one class, and then switch to a different class on the next screen showing the inbound flight(s).

In that case, it most likely means that there is no business class availability. I used the "functionality" just last week - if there are flights available in the higher class (and for me only 2 out of the 8 flights that had economy seats were offered) then they will be offered to you.