It's from the referenced document 'Silent Weapons for Quiet Wars'. It has this descriptive text:
A three industry network can be diagrammed as follows:
A node is a symbol of collection and distribution of flow. Node #3 receives from industry #3 and distributes to industries #1 and #3. If industry #3 manufactures chairs, then a flow from industry #3 back to industry #3 simply indicates that industry #3 is using part of its own output product, for example, as office furniture.