Something goofy is going on with the multi-city revenue booking engine when searching by schedule. When you first enter the booking engine the option to select fare class is not there. You are only given the option to select cabin. After entering the desired flights and clicking the find flights button an error appears that says "The Cabin Requested is Invalid." Once this error appears we now have the option to select fare class. Clicking find flights on this screen will proceed with the booking process.

Screenshots:

First multi-city booking page. Note the lack of fare class option at the bottom, only option to select cabin:


After clicking the find flights button this error appears as do the options to selecting fare class: