What is the "patented removable keel system"/"Interchangeable keel system"?
If you look at the first photo posted in this thread, you can see there is a dark line at the bottom of the keel - this appears to be what they are referring to. It looks like a wearing strip (anodized aluminum?) to prevent excess wear on the bottom of the keels.
That's exactly it: a wear strip
that is easily replaceable
Several HCE-designed cat models, notably those targeted at youth training and club markets, incorporate a wooden and/or composite wear strip. This addresses the excessive wear that GFK hulls experience when operating on sandy beaches. The obvious culprit of this wear is regular, or even just occasional, "beaching" of the cat on a beach, without babying and religious use of beach wheels (starting in the water, not in the sand)!
I personally think the wear strip is a genius feature for anybody but the most serious and speed-addicted competitive sailors. Repairing and/or reinforcing the bottom of fiber-glass hulls is a major PITA!