0.55% carbon actually. I wouldn't make a point of it because dscheidt doesn't need any correction and actual formulation can be inexact; but the designation is slightly important because the threshold for "high carbon stainless" is something > 0.50% carbon. So "X55" is telling you, "a bit more than necessary."
X55 isn't very high carbon by best knife steel standards, but it'll do. The amount of carbon is an important contributor to the ultimate "hardness" of the steel, but there are others just as important -- like hardening treatment(s). Some steel designations are very revealing, the one on the cleaver isn't particularly. Portugal and Spain produce a lot of cutlery for the European market but the brands don't have any cachet here, even though some of them are as good as anything coming out of Germany -- Zanger, for instance. A lot of European pros use Spanish and Portuguese knives -- if that gives you an indication.
If it's a light-duty cleaver of the sort used for stir fry prep, you want a hard steel which can be sharpened to a very actue angle. An X55 would probably require a lot of maintenance to keep sharp while a harder steel could be sharpened more acutely and wouldn't dull as easily. However, if it's more general duty or meat type blade, an X55 might be just the ticket in that it will hold up to abuse and be easily maintained.
Horses for courses,
BDL
