jwillard, bear in mind that, for DesignCAD in 3D Mode, X is left/right, Y is up/down, and Z is forward/backward. This is different from some other programs, where Y and Z may be forward/backward and up/down, respectively. Add to that, in some software positive Z is toward you, while in DesignCAD (in default left-hand coordinates), positive Z is away from you (towards the horizon). Unfortunately, there's no way to change the base XYZ orientations, other than choosing left- or right-hand coordinates to toggle the direction of positive Z as forward or backward.

When you set up a drawing in DesignCAD's 2D Mode for XZ coordinates, the coordinate bar updates to showing X and Z instead of X and Y, but the Point dialogs will show X and Y inputs instead of X and Z. This is cosmetic only, as the 'Y' values you enter actually are used as Z values in this case.

So, with this setup (2D Mode, using XZ plane), drawing a line using 'Pointxy' values of 0,0; 10,0; 10,10; and 0,0 results in a triangle with the hypotenuse going from the origin to 10, 0, 10 (using X,Y, Z values). Switch to 3D mode and it should appear as a flat triangle in the Top view, and simply as a line in the Front and Side views. Gravity-snap to the points in the Main view to verify their locations.

If it's important that you draw in a flat plane only (say at fixed Y) in 3D mode, you can always lock that particular axis to a specified value. For example, if you want to always be sure any points you set lie at Y=0, lock the Y coordinate field in the coordinate bar, then enter 0 as the Y value. Now, even if you gravity snap to some location that has a different Y value (for example, coordinate 10, -2, 10), the point will be set with the snapped-to X and Z values, but the Y value will reset to the locked value of 0.