This controls whether AutoCAD picks the elevation of the snap point, the default being 0 (use the Z value). If you change the variable setting to 1, AutoCAD will use the value of the ELEVATION system variable instead. This can be very useful in an elevated drawing when you don’t want newly drawn geometry to take on the elevations of the existing geometry.
Don’t forget to set it back if you need to snap to elevations, for example when drawing 3dPolylines.