Normally each note has a fixed partial that is used for tuning that note.  But you can make TuneLab decide which partial is the best to use when the note is played.  On this page you can choose which notes will be subject to automatic partial selection, and which partial is the highest partial that should be considered for automatic selection.  If a note is in the range you select, then when you first play that note, TuneLab will find the strongest partial in that note and switch to it.  TuneLab will not switch partials unless the new partial is at least twice as strong as the current partial.  Once the partial selection has been made, TuneLab will not try again to select a better partial until a new note has been selected.

You can also turn on the "Persistent Partials" switch in the main Settings if you would like these automatic partial selections to be stored in the current tuning file.  Otherwise the automatic partial selections are just temporary, and the partial selection will revert to the current table of partials when a different note is selected.