The Calculate button on the Plane Offsets dialog box (Insert | Feature | Constructed | Plane) allows you to calculate the X, Y, and Z nominal values from new offsets entered.
For example, suppose you construct an offset plane between three circles (CIR1, CIR2, and CIR3). After you select the features and click the Offsets button, you have X, Y, and Z nominals of:
X = 6
Y = 2
Z = 0.95
If you change the offset values by 3.0 for each circle and click the Calculate button, the X, Y, and Z are updated to:
X = 6
Y = 2
Z = 3.95
If you then click OK and construct the offset plane, you'll notice that the newly constructed plane lies 3.0 units higher in the Z axis.
Plane offsets only apply the offset in the Z axis.