Draw Mode and Sculpt Mode
The Drawing Planes selector helps to select the plane in which the newly created strokes are drawn.
To see which plane you are using when drawing strokes, you can enable Canvas in Viewport Overlays. See Viewport Display to know more about Canvas settings.
The Drawing Plane selected has effect only for new strokes and does not affect the existing ones.
Strokes are drawn on the plane determined by the XZ axes (front view).
Strokes are drawn on the plane determined by the YZ axes (side view).
Strokes are drawn on the plane determined by the XY axes (top view).
Strokes are drawn with the current 3D Viewport orientation.
Strokes are drawn with the current 3D cursor orientation.