Race scheduler is a tool made to help you with planning your trainings. It comes in two different modes.
If you select Races for a selected character, you will be recommended what optional races could be a good fit for your character, taking things like aptitudes and conflicting objectives in account.
In the Characters for selected races mode, you will be able to select races you need to run, and the tool will recommend you which characters are able to do it. Good for planning things like achievements and limited missions.