Forward or Backward Pass

[bwilldrd]

Bear with me,

Players A and B are running down field (already past the line of scrimmage). B trails A by 2 yards. A passes the ball to B. The direction of the pass (relative to passer) is clearly backwards, but because both players are running, the flight of the ball is forward.

Basically, is forward/backward judged relative to players or relative to marks on the field?

Does the answer differ in HS, NCAA, NFL?

Thanks