When you select an unequal temperament to apply to a tuning, the offsets that define the temperament can be optionally normalized according to the settings of the switches on this screen. "Normalize for A=0" will shift the entire temperament (all 12 offsets), if necessary, so that the offset for A is zero. (All the temperaments supplied with TuneLab are already normalized to make A=0, so this option normally is not needed.)

The second normalization option is "Normalize for Avg.=0". This will shift the entire temperament so that the average of all 12 offsets is zero. This is useful to prevent an overall shift in the average pitch of the piano, which would make the piano harder to tune. If neither of the normalizing options is switched on, then any selected temperament would be used "as is".