The external Id allows API partners to store their unique Id on a proposal and correlate it back to a sold order. The API partner needs to create a link out from their system to AdCellerrant, which will allow a user to complete a proposal in AdCellerant.
The API partner can correlate the external Id saved on that proposal back to the original order within their software application. The <external_id> would like the unique Id with the API partners system.