Street Sign Brackets for "U channel" posts

What does parallel mean? What does extruded mean?

Extruded means that the bracket is for a sign with a extruded edge. See picture at top of... https://www.signsdirect.com/Home/Street-Name-Signs

Parallel and perpendicular refer to how the sign will orientate to the "U" shape in a u-channel post (when viewed from above). In either case, the sign will be parallel with the ground. In one case it will be perpendicular with the top edge of the "U" shape of a u-channel post and in the other case it will be parallel with the top edge of the "U" shape.