[responding to "epaalx"] As Jason wrote in an earlier comment, the term "HUD" was inspired by the motion picture "The Hudsucker Proxy". The "HUD chain" is essentially a logical stack of filter layers stacked up on either side of the dual-proxy processing paradigm used by the TMM (the Traffic Management Microkernel process). An input packet comes in and goes up the HUD chain on one side (potentially undergoing processing at each layer of the stack), and then from the pinnacle goes down the chain on the other side and then (typically) out to something outside BIG-IP.