steel shot (tungsten, heavy, fasteel, or the like) will be clearly marked.
the numbers relate to the size of pellets the smaller the number the larger the shot size and the fewer there are in each shell. i know a lot of people like the 3 in and and 3.5 in shells and they do an excellent job. i prefer the 2 3/4. if i am hunting an area for just ducks i usually shoot a 4 shot and will fall back onto a 2 shot if i run short of shells (i buy most of my shells during the off season). i will shoot a 2 shot or BB shot if hunting areas that may present the oppertunity for ducks and geese.