What is strong flour? I have a bread recipe that calls for white flour, whole wheat flour, and strong flour.

OK, I picked up a jar of "lemon salt" thinking it might be something other than it is. Anyone know what it is? And anyone have any ideas what I could do with it? One tiny taste of the stuff raw is enough to knock you into a permanent pucker, so whatever you do with it, I'm sure you'd only use a bit...