In Touch with Dynamics (GP) keeps finding required fields for working with Word Templates. Their latest revelations are at Dynamics GP 2010 Word templates (more required fields SOP Blank History Invoice)
Dynamics GP 2010 Word templates (more required fields SOP Blank History Invoice)
Excel & OfficeSales, Purchasing & InventoryGP 2010
Archived post from 2011. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.