440 A - 440 B - 440C
The carbon content (and hardenability) of this stainless steel goes up in order from A (.75%) to B (.9%) to C (1.2%). 440C is an excellent, high-end stainless steel, usually hardened to around 56-58 Rc, very tough and with good edge-holding at that hardness. 440C was the king of stainless cutlery steels in the 1980s, before ATS-34 took the title in the 1990s. All three resist rust well, with 440A being the most rust resistant, and 440C the least. The SOG Seal 2000 is 440A, and Randall uses 440B for their stainless knives. 440C is fairly ubiquitous, and is generally considered a very good general-use stainless, tougher and more stain resistant than ATS-34 but with less edge-holding and weaker. If your knife is marked with just "440", it is probably the less expensive 440A; if a manufacturer had used the more expensive 440C, he'd want to advertise that. The general feeling is that 440A (and similar steels, see below) is just good enough for everyday use, especially with a good heat treat (we've heard good reports on the heat treat of SOG's 440A blades, don't know who does the work for them). 440-B is a very solid performer and 440-C is excellent
(aka 6A 8A 10A)
Japanese stainless steels, roughly comparable in carbon content to 440A (AUS-6, .65% carbon) and 440B (AUS-8, .75% carbon) and 440C (AUS-10, 1.1% carbon). AUS-6 is used by Al Mar, and is a competitor to low-end steels like 420J2. Cold Steel's use of AUS-8 has made it pretty popular, as heat treated by CS it won't hold an edge like ATS-34, but is a bit softer (and therefore weaker) and tougher. 8A is a competitor of middle-tier steels like ATS-55 and Gin-1. AUS-10 has roughly the same carbon content as 440C but with slightly less chromium, so it should be a bit less rust resistant but perhaps a bit tougher than 440C. It competes with higher-end steels, like ATS-34 and above. All 3 steels have some vanadium added (which the 440 series lacks), which will improve wear resistance and refines the grain for both good toughness, and the ability to sharpen to a very keen edge. Many people have reported that they are able to get knives using steels that include vanadium, like 8A, sharper than they can get non-vanadium steels like ATS-34.