I found problem in setting cross-company transaction. 
What I have done are as follow: 
1. Create material master Z for plant A (company A), and plant B (company B). 
2. Create vendor masteri for company A, assigned to plant B; e.g: vendor X assigned as plant B. 
3. Create PO (doc.type NB) from plant A to vendor X for material Z. 
4. Supposedly system would automatically recognised this as cross-company transaction. Then plant B could create delivery refering to this PO. 

In my system, when I try to create the PO, SAP generates error message said that Sales and Distribution Data haven't been maintained for material Z, which I have already done. 
Could anyone help? Recently I've got chance to look at the system where the cross-company has been already set. By comparing the PO, I knew that in the successfull cross-company PO, there are additional tab in item detail named Shipping. In this tab, there is information about customer, delivery type used, etc. Which make me wonder, is it the source of the problem. Should I set the customer master data refering to the plant issuing the PO? If I should where could I set this? 

You need also to configure your Stock Transport Order settings for your Cross-Company Business Transaction to work. 

1. Customer No. for the Goods Receiving Plant - OMGN 
2. Availability Check- Checking Rule (if necessary) - OMGN 
3. Assign a Delivery Type for the Delivering Plant - OMGN (for Stock Transport Orders, NLCC) 
4. PO type (which i belive you have done) - OMGN 
5. Assign Vendor No. to the Supplying Plant (done) - VK02 
6. Assign Customer No. to the Purchasing Plant for the Inter-Compnay Invoice (but you need to assign this to the Sales Organization pre-assigned to the Purchasing Plant), 
IMG-SD-Billing-InterCompany Billing-Define Internal Customer No. by Sales Org 
***and by the way for the Invoice to work between Cross-Compnay Plants, you need also to have a Sales Org for the Supplying Plant and a Pricing Determination Procedure. 

I've actually done this already and it should work. 

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