A reference plane is a flat geometric guide used to describe placement, alignment or movement in a 3D scene.
Two directions define a plane
In a three-axis system, XY, XZ and YZ are the familiar coordinate planes. Each uses two axes while holding the third coordinate fixed. The plane that behaves like the ground depends on the application’s up-axis convention. A reference plane can also have any other orientation, such as the tilted face of an exhibition panel.
Three things that can look similar
A guide plane in an editor, a rendered rectangle and a detected real-world surface are different things. The guide helps you work; a mesh can be visible in the final image; a detected plane is an estimate supplied by an AR system. A mathematically infinite plane may be displayed as a small patch simply to make it visible.
An artwork as a reference
When composing digital content around a painting in Painta, it is useful to think of the artwork surface as a reference plane. Content can sit on that plane, in front of it or behind it. The tracking system and viewer determine the actual alignment. Treating the plane as a conceptual guide does not imply that a physical wall has been scanned or that it can hide virtual objects.