The Masazumi steel is carbon, aka, non-stainless. It is Swedish, which generally means a higher level of purity, and that they take a finer edge, but the real big kicker I find with Swedish steels is that the high level of purity allows them to be hardened a little bit harder without becoming chippy.
This steel is comparable to Masamoto HC in my opinion, and I'd even dare to say it's the same alloy. Really good stuff, I could grind it a little bit and maybe give you my opinion on carbon content, but then again I believe the knife I have is just a tester and needs to go back. I could always round the spine and see what happens. (BTW, in case you didn't know, when grinding carbon steel it will make a shower of sparks, unlike stainless which barely sparks, and the more carbon you have will generally make longer sparks, not to mention the 'pop' really cool at the end, like a sparkler) I doubt I'm truly good enough to pinpoint it yet, but I hear that experience blacksmiths can tell you quite easily. I do have experience with a few steels, just not a bunch (yet).
So, expect this steel to take high refinement (Mine took 4k like it was nothing) and a very steep edge (10 degrees per side). It has good toughness as well. If I had to compare it to a Hitachi steel, I'd say it's probably a little bit better than Blue #2, or at least ballpark.
And BTW, AEB-L doesn't just take a good edge; Along with White #1 you can argue it takes the BEST edge. (yeah Adam, I did that for you!)