Are you working with the latest release of Adagio Invoices? The term used and the import field name in AI 9.0B is Hdr-New invoice. I think that Hdr New Header Character is the same field name for previous versions. Its purpose is to signify whether the record is the start of a new invoice or an additional line to another invoice.

You should be working with at least AI 9.0A because that is when the import function changed dramatically. Importing to any pre 9.0 version of AI is very challenging. Softrak did us all a great service by making the AI import completely template-driven in version 9.0A