1.What should be the settings vendor company code for creating PO from auto purchase requisition being created after saving sales order from customer 1000 to Ccode ABCD. And after doing dummy MIGO for the above mentioned PO, how to send invoice to customer 1000 in MIRO.
You need to use item category as TAS in the sales order of the ordering company code ABCD. You may not need to do dummy MIGO unless otherwise dictated by the business. The ordering company ABCD can seek any shipping notification from supplying company code and based on that can create customer invoice (sales order related invoice).
2. Is it correct to assign plant EFGH on the third party sales order or I should create dummy plant for Ccode ABCD.
This is correct, you can assign plan EFGH in Third party sales order.
3. How ccode EFGH would be able to ship items to customer 1000 W.R.T PO. Do I have to create sales order to EFGH with reference to PO?
In the purchase order that company ABCD creates, the delivery address should represent customer address.
4. What should be settings for intercompany customer for intercompany billing.
Since delivery between company codes is not made, you need to refer OSS Note 63459 - Order and third-party-related intercompany billing
The setting is similar to any regular intercompany scenario.The only difference is billing part, which is given by the above OSS note.
Regards,