why VBFA is needed if I have VBAP, VBAK.

please explain what is this subsequent(VBELN) and precendent(VEBLV) documents in VBFA.

How they are related to each other.

I also didn't understand the following code related to VBFA:

LOOP AT it_so.

SELECT SINGLE name1 FROM kna1 INTO v_name

WHERE kunnr = it_so-kunnr.

SELECT SINGLE vbeln FROM vbfa INTO v_delnum

WHERE vbelv = it_so-vbeln

AND vbtyp_n = 'J'.

IF sy-subrc = 0.

SELECT SINGLE vbeln FROM vbfa INTO v_invnum

WHERE vbelv = v_delnum

AND vbtyp_n = 'M'.

ENDIF.