The full add-on: all five measurements, the viewport overlay and Copy, both zips (Blender extension and legacy add-on), the documentation and free updates for 1.x. GPL-3.0-or-later, no account, no trial, no limits.

No: Blender's default scene counts one unit as one metre, so the 2 m default cube is 2000 mm. If your model came from an STL in millimetres, set '1 Blender unit =' to '1 mm' in the Caliper tab, or set Scene > Units to Metric with unit scale 0.001.

A round hole in a mesh is a polygon. Its corners sit on the 8 mm circle, but the flat sides are closer to the centre, so a 32-sided 8 mm hole is 7.961 mm across the flats. That is the size that prints (often a bit less again because of the printer). More segments bring the two numbers closer.

No. Every measurement uses the object's scale and rotation, so you see the real world size. Non-uniform scale on a round hole turns it into an oval; Diameter then gives the best-fitting circle and says 'Not round' as soon as a vert is more than 1% of the radius off that circle (from just over 2% scale difference).

Yes. Select both objects, press Tab for multi-object Edit Mode and select one vert (Distance) or one face (Gap / Thickness) in each. Object Size measures all selected objects together.

No. It only reads the selection and the geometry. Undo works as usual; the result lives in the panel and the overlay.

MM Caliper measures. Precision Holes & Inserts is our separate, paid add-on that builds holes, insert pockets and nut traps with printer compensation. Many people use MM Caliper to check what Precision Holes made.