Mechanical Offset/ Height over Bore
This is basically the physical difference between the center of the optic and the center of the bore.
All optic based weapons have mechanical offset.
It matters at closer distances.
I just saw another super tall mount release.
It is the users responsibility to master offset.
Below is the offset with live fire of a Unity FAST mount, Aimpoint CompM5s on a 11.5" 5.56 at 7yds. The red dot is placed high and the rounds impact low.
Mechanical offset.
Know it.