AP 9.1 - How does AP MATCH process identifies which receiver lines are for 2 way matching ?

I understand that when we are selecting RECV_REQ as "optional" for a line, while creating a Purchase order, that line undergoes 2 way matching. The same is reflected in RECV_LN_SHIP.MATCH_LINE_FLG field also (when N -- matching is not required, when Y -- matching is required).

But could you please help me in identifying which code/SQL identifies in AP_MATCH App engine, whether a receiver line should be or should not be considered for matching (whether 2 way or 3 way).