The GP Support and Services Blog has Information regarding Encumbrance and Blanket Purchase orders
Information regarding Encumbrance and Blanket Purchase orders – Dynamics GP Support and Services Blog – Microsoft Dynamics GP – Microsoft Dynamics Community
Microsoft & the GP EcosystemSales, Purchasing & Inventory
Archived post from 2013. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.