The 3 Dimensional option in the Distance dialog box (Insert | Dimension | Distance) calculates the three-dimensional distance between two features.
If one of the input features is a line, centerline, or plane, PC-DMIS computes the 3D distance normal to that feature.
If both features are lines, centerlines, or planes the second feature is used for the datum. If both features are planes, the 3D distance is the perpendicular distance from the centroid of the first plane to the second plane; the second plane acts as the datum.
If neither input feature is a line, centerline, or plane, PC-DMIS computes the shortest distance between the two features. (Verify that the mode is set to 3 Dimensional.)