I learned about the system variable OSNAPZ from Lynn Allen’s Circles and Lines blog.

When snapping to 3D points AutoCAD will grab the x, y, and z. However there are times when you don’t want it to grab the z value. To do this, type OSNAPZ on the command line and set it to 1.

One use for OSNAPZ would be that you could find the 2D distance between 3D points. To find the X-Y distance between two 3D points, set OSNAPZ to 1 and type the DISTANCE command. AutoCAD will ignore the Z value and only give you the distance in 2D.

Another use would be drafting in 2D while snapping to 3D points. Have you ever noticed your linetypes start doing strange things when you snap to points with different Z values? To solve this, set OSNAPZ to 1.