you probably only did the setup to allow it in ORDERS but not INVOICES.  go back to the setup and double check.  

Thanks.Actually is only in invoices, not in orders.

do you have it set to AUTO-INSERT?

Yes.

What is the purpose of the optional field?  When you say 'The field is not in the optional field list when we process invoices.' what do you mean by that?  What does 'Process invoices' mean?

The purpose is to flag invoices that need to be included in a query. It is a yes/no optional field. When the OE invoice option is used, in the optional field section the optional field is not on the list.

Then you didn't set it up correctly.