I have sitting in my home everything from plain "white chocolate" to milk chocolate with 35% cocoa content, to dark varieties at 53%, 62%, 88%, and even "pure" unsweetened 99% baking chocolate. Like a lot of people, I'd always assumed that these percentages referred to cocoa solids. Naively, I'd not realized that "cocoa content" can legally refer to both cocoa solids and cocoa butter. Let me explain with a couple of visuals.
Let's say these are both 70% chocolate. Nice and dark, right? The houndstooth represents the sugar content, at 30%. The polka dots represent the cocoa solids, and the white space represents the cocoa butter. In each case, we have 70% cocoa content. But one of them might have something like 15% cocoa solids, while the other is only 15% cocoa butter. And yet, both are considered dark. It's kind of a dirty marketing trick, isn't it? Even dirtier than me using houndstooth and polka dots in the same image.
There is something else interesting about this. As it turns out, the darker the chocolate, the more structural integrity it has. This is pretty important when you get into things like chocolate sculptures. White chocolate is a lot more versatile in terms of color possibilities, but you would never want to use it for base structure in a large sculpture. Milk chocolate will give you a little more strength, but dark is going to be your best bet.
One last thing that I should mention. The higher the ratio of cocoa solids to cocoa butter, the more viscous the melted chocolate will be. If you have a keen enough eye to pick out which brands are thicker when melted, then you can use this as an indication as to what kind of ratio of cocoa solids to cocoa butter you have.
I wish I had more knowledge about this, but I'm still relatively new to the chocolate scene. I would invite any insight or links that anybody passing by might have to offer me. But I thought I'd throw this out while I was thinking about it.