Normally, when adding a new line to a purchase order (either by typing in a part no. or by clicking on the part no. lookup button), it is treated as a normal inventory item. To add a non-inventory item, the user has to right click and explicitly choose "Insert Non-Inventory Item".
This plugin reverses this behavior - when a line is added to a purchase order it is a non-inventory item by default. Normal inventory items can only be inserted by right clicking and choosing "Insert Inventory Item".