Hello Ankit,
>> 1) Assign Org Unit to Plant -
Once we maintain the Delivering Sales Area to Delivering plant it reflect to table-LIKP and when system creates the inter company invoice with reference to Delivery system take the reference of this sales area.
Additionally we see the VPRS reflect to intercompany invoice only to it increase the revenue of delivering company code/sales org and customer is nothing but ordering company code/sales org.
>> 2) Define Internal Customer by Sales Org.
Yes, your understand is correct. 
I hope now picture would be clear about inter company
