The PFB patch is a jamming thread locking (Polyamide patch). The coating for Type K or KN (standard spring load) is indicated blue, for Type KS or KSN (high spring load) green. For this Type of thread locking a relatively high torque is required. Therefore this version with internal hexagon is more preferable than the version with slot (GN 615).
MVK (Micro encapsulation) is a gluing thread locking (indicated red).