A "vertical plane" refers to an imaginary flat surface that is in an upright or perpendicular position, going straight up and down. It is used to describe a continuous area that is vertical or in a straight, upright position. Full definition
Olympic lifting with a barbell not only requires precise technique but forces trainees to follow a path along the set vertical plane of the lift. (onnit.com)
I have long thought it a great sculpture, with its slightly inclined vertical planes, which, if projected, would meet a thousand feet above the earth's surface. (theguardian.com)
Google's ARCore supports two types of horizontal planes (horizontal upward facing and horizontal downward facing), but as far as we know, it does not support vertical planes. (theverge.com)