nearly vertical. It needs 12-15 degrees from vertical. Horizontal is fine.

There are 2 Maslow versions: V1 was hanging from 2 chains, and "4" is held from the 4 corners of the workspace, allowing it to be used both in the vertical and horizontal orientation.

V1 also had the electronics & motors on the frame, while V4 moved everything on the sled itself. This enables the option for the "frame" to be basically just 4 anchors on your garage floor, which makes this a very flexible machine.

In the about link (above), they do mention the frame:

"If you would like to build the frame shown in the video, that frame is 12' feet, by 2’ by 7’ tall."

Yes, they support either setup, vertical or horizontal