On the additional Settings page you can apply an historical or non-equal temperament to the current tuning. Non-equal temperaments are defined by the 12 offsets which are applied to every octave in the scale. These offsets are in addition to the offsets due to stretch, and the overall offset discussed earlier. For example, in the Villotti-Young temperament the offset for the note D is +1.95 cents. This offset applies for all the D notes in the Villotti-Young temperament. So if you know the offset for the 12 notes of the chromatic scale, you know the offsets for any note.
Before you can enter any offsets you have to enable historical temperaments by toggling the "Enable non-equal temperament" switch at the top of the screen. When that switch is turned on, 12 text fields will appear on the screen. They are arranged in three rows of four each, starting with 'A' in the upper left. As you cycle through the fields, if VoiceOver is enabled it will announce the note for that field as well as current value entered. Enter the 12 offsets for the temperament you have in mind using the normal VoiceOver interface for text fields. When you exit from that page the temperament defined by those 12 offsets will be in effect as you tune. To remind you that a non-equal temperament has been selected, the tuning file name will be announced with the phrase "Unequal temperament enabled" added on to the end of the tuning name.
The 12 offsets can also be saved in a tuning file. If you have made a custom tuning file and entered offsets for an historical temperament, those 12 offsets will be saved along with other tuning data when you save the tuning. The VoiceOver announcement of the tuning name on the main tuning screen will also contain a reminder that an historical temperament is in effect, so you do not accidentally tune a piano in an historical temperament.
The most recent 12 offsets entered or read from a tuning file remain in memory even when historical temperaments are turned off. This can be very useful in managing several temperaments. Suppose that you have an historical temperament entered and stored in the tuning named "Steinway." Now suppose you want to apply that temperament to another tuning which you have stored and named "Baldwin." If "Baldwin" does not have an historical temperament associated with it, then when you load the "Baldwin" tuning, the 12 offsets for unequal temperaments remain unchanged in memory, even if they are not enabled or effective. So first load the "Steinway" tuning, which will set the 12 offsets according to the temperament associated with that tuning. Then load "Baldwin" and go to the Secondary Settings page and enable non-equal temperaments. This will attach those 12 offsets from the Steinway tuning to the Baldwin tuning, which you can now save. But if "Baldwin" already had a different historical temperament, this would not work because the "Baldwin" temperament would replace the "Steinway" temperament in memory when it was loaded. In that case you can load "Baldwin," turn off non-equal temperaments, and save "Baldwin" to remove the temperament from that tuning before loading the "Steinway" tuning.