Don,
N690 Cobalt Stainless is an Austrian Steel that has the added ingrediant of Cobalt. Cobalt enhances the quality and the performance of stainless steel (provided the steel has certain ingredients with specifiic percentages...and this steel does!) by allowing the steel to be very hard (58-60 Rockwell) but not brittle, very resistant to any type of corrosion, and the abilty to hold an edge even after hard use.
This steel is rarely used in a production knife due to its cost...it is very expensive. It is normally used by custom makers.
N690 is used in the meat cutting industry as they need a blade in their automatic cutters that does not corrode with blood and salts and does not need to be sharpened often, It is also used in the medical field for surgical implants and surgical blades.
From a practical point of view I feel that N690 Cobalt is unsurpassed as a blade for use in a "tactical" knife.
Hope this helps.