questions about OBCD

Hello experts,

After checking, I still got some questions. Pls advise. Thanks.

Business flow: sales order -> outbound delivery -> invoice (F2) -> Intercompany invoice (IV) -> AP document through auto-posting.

Questions:

1. If using the standard customizing, why input tax rate should be the same as output tax rate?

My testing: In OBCD, output tax rate is 10% while input tax code J8 (stands for 10% input tax rate). Details in the attachment. It works fine. If J8 is changed to J0 (stands for 0% input tax rate), auto-posting failed with error:

Tax entered incorrect (code J0 , amount 1.36 ), correct 0.00 CNY

Message No. FF707.

2. How to determine which tax type should be entered in OBCD?

3. One entry is added to OBCD: tax rate is 0.000 while Tx is J0. Output tax condition record is changed to 0. I used the same customer and material, but auto-posting failed with error below. Why?

Assigning a new tax key for X0 and VAT 0 not possible

Message No. FD008