The Angle box in the Angularity dialog box (Insert | Dimension | Angularity) enables you to type the nominal angle from the datum feature. This is the angle between the two features. PC-DMIS then calculates the deviation of one feature from the angle.
Clicking the Find Theo Angle button calculates the nominal reference angle you should use from the theoretical values of the selected features. This makes it so you don't have to go and look up the angle in the Edit window.