What does UK+... AND ÖK+... mean?

[Abbokar]

Hi!
I am wondering what "underkant" and "överkant" mean and how to interpret them? For example, "underkant" on an opening or hole or on a K-drawing?

 

[Henrik.woll]

Yes, it is as it sounds, the underside or top side of something. Then there is usually, for example, ÖK+xx. Where +xx is the specified height from an established reference object.