On this screen you can set up which notes you want to measure for inharmonicity when a "new tuning" is selected.  Tap on the spot for any note to toggle between 'M' and '-'.  Notes marked by the 'M' are the ones you want to measure.  Typically you would measure 5 or 6 notes.  For example, you could measure C1, C2, C3, C4, and C5.

When a "new tuning" is selected from the Files menu, TuneLab will switch to the lowest note marked with an 'M'.  It will not automatically start the measurement. You  still have to do that by tapping on the measure button, shown here.  After you Save the first measurement, TuneLab will automatically switch to the next note marked for measuring with an 'M', and so on.  At each note, you only need to tap the measure button just before you play the note for a measurement. If you have enabled the fully automatic mode for the tuning curve adjuster, then an automatic stretch adjustment will be made after each inharmonicity measurement.

If you wish to create a Split-Scale tuning, then measure the inharmonicity for the indicated notes as usual, but afterward manually select the two notes on either side of the break and measure their inharmonicity too.