3D Systems CubePro Manual page 10

Incorrect Closed Z-Gap
If there is a gap between the Closed Z-Gap bars (B) and the baseline (A), that print jet is
too high and should be adjusted. Once it has been adjusted, print the calibration file again
to verify that the print jets are level.
Correct Open Z-Gap
If there is a gap (B) between the Open Z-Gap bars (C) and the baseline (A) and there is no
gap between the Closed Z-Gap bars and the baseline, that print jet is level.