The line is the eyeline. If you’ve looking up - like a highway billboard, it’s ATL. If you’re looking down - at a brochure- it’s BTL. Of course this definition

The line is the eyeline. If you’ve looking up - like a highway billboard, it’s ATL. If you’re looking down - at a brochure- it’s BTL. Of course this definition is not exactly correct or exhaustive anymore, this is just the origin. Now it’s understood as defined by the other commentators.