SAP Function Module search on WSSB_
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WSSB_MAP_VBRKVB_TO_BILLHEAD - Map VBRKVB to external structure of billing head
WSSB_MAP_VBRPVB_TO_BILLITEM - Map VBRPVB to external structure of billing item
WSSB_RFC_BILL_DOC_READ - SRS:RFC for billing doc detail read
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