When you select ‘Search by Price’, you will see a list of flights for the dates you selected, arranged by fare. When booking return fares, you also have the option to ‘Search by Price (dates flexible)’, in which case you will see the best available fares for the dates you selected, as well as nearby dates.
When you select ‘Search by Schedule’, you may select each flight in your itinerary individually, to best suit your travel plans.