On the Partials Settings Screen is where you can select functions relating to which partials are used for tuning each note.
At the top of the screen is a switch that controls the automatic selection of partials while tuning.
Below that is a button that takes the current table of partials from the current tuning and stores that table as the default partials to be used when you create a new tuning. For example, if you want to make the lowest octave in a new tuning use the 5th partial instead of the 4th partial, select each note in the lowest octave one at a time and for each note change the partial to be the 5th partial. Then come to this page and tap the button to save the current partials as the future default for new tunings.