What is difference between union and region form Auto CAD point of view?

Union command for3d objects. region  applied on2d .

Union command is used to combine3d solid objects. therefore region command is applied on2d objects.

UNION (Command) Combines two or more3D solids, surfaces, or2D regions into a single

REGION (Command) Converts objects that enclose an area into a2D region object. Regions are2D areas that you create from closed planar loops of objects