The main referance for measuring vertical distortion in a frame or unitized body is called the:

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A fixed baseline is used to gauge how far each point on the frame has moved up or down. The datum line serves as that constant reference, so vertical measurements of the frame’s points are taken relative to it. Because distortion can change many spots differently, comparing them to this unchanging line shows exactly where and how the frame is warped, sagged, or twisted. Technicians establish the datum line from known reference points or alignment fixtures, then record vertical offsets to assess straightness.

The centerline is mainly about the vehicle’s longitudinal symmetry, not the vertical deviation from a fixed baseline. A reference axis is a general term for a line used as a reference, but in this context the standard, precise term used for vertical distortion checks is datum line. A leveling line relates to ensuring surfaces are level, not to diagnosing distortion of the frame itself.