The Calculate button calculates the X, Y, and Z nominal values from new offsets entered.
For example, suppose you construct an offset line between two circles (CIR1 and CIR2). After you select the features and click the Offsets button, you have X, Y, and Z nominals of:
X = 4.5040
Y = 3
Z = 0.1582
If you change the offset values by two for each circle and click the Calculate Noms button, the X, Y, and Z update to:
X = 4.5040
Y = 5
Z = 0.1582
If you then click OK and construct the offset line, you'll notice that the newly constructed line lies two units higher in the Y axis.
Only the Y axis is offset for line offsets.