Why couldn't you use a Column-Post Validate event on the ItemCode field to fill the PO No. field for your conditions. It would only fire once per ItemCode and not when otherwise changing the line. 

connex Sorry but I am not following you on this code wise.

connex Sorry but I am not following you on this code wise.

I said you can't prevent an event script from running more than once, but what connex is suggesting is using an event trigger (column post-validate on ItemCode) that naturally will only run once per line.  After you set a line's ItemCode, it never changes.

If your business logic can be properly applied at that point, it is an excellent idea, and easier than the checkbox.

I simplified the script without the checkbox and it populates the purchaseorderno field with the "C", however it does it no matter the customerno, so I have something wrong there.  Also curious on if I want the "C" value to be set only if a PO is not created, the column post-validate with this script fires off right after you enter the item code, I would think we would need it to look at the field after the record was written, gives the user a chance to create a PO if needed without going and deleting the "C".

If oSession.CompanyCode = "ROI" then

nCustomerNo = ""
nVendorno = ""
nPOno = ""

rVal = oBusObj.GetValue("CustomerNo$", nCustomerNo)
rVal = oBusObj.GetValue("VendorNo$", nVendorNo)
rVal = oBusObj.GetValue("PurchaseOrderNo$", nPOno)

	If nCustomerNo <> "2861000" or "2303006" then

		If nVendorno = "0001226" then		

			If nPOno = "" then			
			rVal = oBusObj.SetValue("PurchaseOrderNo$", "C")
			
			End if	
		End if
	End if
End if

The business process is up to you.  For a pre-write event on the detail table you'd need the checkbox for control.

Use oHeaderObj for your GetValue on CustomerNo (since it is a header field).

Your filter on nCustomerNo is all wrong (and I'm surprised you don't get a VBScript syntax error).  Try something like this:

variable <> "A" and variable <> "B"