All recipes in the Invention System belong to a certain "Drop Pool". This is the only way to get each recipe without purchasing them from a Merit Vendor; though note that some recipes cannot be purchased at a Merit Vendor. In the case of "Pool C" and "Pool D" recipes, these were originally awarded at the end of Task Forces, Strike Forces, and Trials. "Pool C" recipes were awarded for Task Forces, Strike Forces, and any hero-side Trials that were not Power Respecification Trials. "Pool D" recipes were awarded for Respec Trials and Hamidon Raids.