Meaning of this code

Dear Experts,

Can somebody please explain me the meaning of the following code? I understand most of it. But things like DOC_TO_LOOK- what does it do?

• Pickup clearing document into internal table.

SELECT * FROM BSEG INTO TABLE ZCLEAR_DOC

WHERE BUKRS = FS_DTFIAR_3-BUKRS

AND BELNR = FS_DTFIAR_3-AUGBL

AND GJAHR = ZFISYEAR.

• Calculate amount to clear by looking for residual items.

• Find original invoice: Invoice reference or current item.

if FS_DTFIAR_3-REBZG = ''.

DOC_TO_LOOK = FS_DTFIAR_3-BELNR.

else.

DOC_TO_LOOK = FS_DTFIAR_3-REBZG.

endif

Thank you very much.