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I’m trying to superimpose 2 Stl scans from an intraoral scan and want to know the deviation between the two scans.
Is there any other unit other than the RMS?
What is the unit of rms error is it micrometer or millimeter
Thanks!

The units of the distances are the same as the coordinates of your STL files (and sadly this information is not contained in an STL file, you have to derive it from the scale of the object).

And apart from the RMS, you can after the registration compute the distances between the 2 entities, and maybe compute the average and standard deviation, the max distance, etc.