Hello Arjun,
Not sure if I get your question correctly. The association of the document header and item is linked using table /SAPSLL/CUIT, and before that table /SAPSLL/CORREF will store the relationship of the reference document number and the guid key of the customs document header being used in GTS.
So in conclusion, the process looks like below:
/SAPSLL/CORREF, you get the guid of the customs document header
/SAPSLL/CUIT, you can get the document items of this header
/SAPSLL/LEGCON, the relationship of which license is being assigned to the item
/SAPSLL/LCLICAQ, get how many quanity left for the license using guid_lclic.
Hope it clear your thoughts.
Regards,
Derek