when i have 2 delivery doc.with different PGI dates and when i create a billing doc. for both together .which date (for billing) the system will take.
the latest pgi date or current date
There will be invoice split and two billing documents will be created. However, you can avoid this.
When you are doing collective billing you can default the billing date. In VF04 transaction, go to the default data tab and enter the billing date as required by you. The split which was happening due to different PGI dates will not happen now. Even in VF01 also you can enter the billing date so that invoice split doesnt happen.