invoice header and item conditions

hi all ,

i have doing a invoice layout :

i got KNUMV from VBRK through invoice document nr , and get all the records from KONV through KNUMV. but i don't know how to distinguish header conditions and item conditions. the item condition i suppose is the records with KONV-KPOSN = invoice item number , but where should i get the header condition , or i should calculate myself , if so , how to calculate ?

please guide me !

Best regards

Kevin