Chocolate: difference between "Cocoa Mass" and "Cacao"?

Yep, cocoa and cacao are the same thing.

The 72% has sugar making up the rest of the mass. The description on amazon actually mentions that it's 27% sugar and 43.5% cocoa butter. The rest is cocoa solids, the chocolate-y stuff. As you say, that particular brand is pretty expensive; it's also pretty popular and well-regarded. Since it has plenty of sugar in it, you can use it for pretty much anything, including things like coatings, or even just eat some.

The 100% is unsweetened (there's no room for any sugar in there) so it's really only usable for things like baking, where you'll add some sugar. It's unlikely to be pleasant to eat on its own.

For what it's worth, usually in English you'll hear cacao only when referring to the overall cacao seeds and their use as a component of chocolate. It's not common in non-culinary contexts. For example, we'll say "cocoa powder" not "cacao powder".

As for the difference in labelling, there is no difference, as Jefromi already said. I could imagine some producers calling more-or-less raw liquor "cacao" and the processed product "cocoa mass", but this is not standard usage.

While there is no difference between the words, the product may still be very different. The problem is that cocoa mass with the butter removed is still called cocoa mass, just like skim milk is still called milk. Confections made with low-fat cocoa mass taste much worse than ones made with full-fat cocoa mass, but there is no way to determine what you have from the package. Valrhona is a pro brand and it does contain enough cocoa butter for great confections, but there are cheaper brands which sell full-fat too. You have to try the cheap one to see if the quality is good enough.

You also have to see what your recipe requires to decide which one to use. You need 100% for recipes calling for "bitter", "dark" or "cooking" chocolate. 72% is considered in the "semi-sweet" range in English books. In other cultures, it is different, with chocolates as low as 50% being called "bitter".

Cacao is actually the name of the tree. The beans in the Cacao pods are fermented and processed to make chocolate. Pure chocolate has two main components: cocoa solids and cocoa butter.

Cocoa mass listed on labels contains both cocoa butter and cocoa solids. Some manufacturers list the percentage as cocoa or cacao (as refered to in some non-english languages) which also typically refers to cocoa-mass.

The balance of the percentage listed (100-72 = 28%) usually is sugar but some manufacturers pull back the percentage of cocoa butter and add oils (e.g. palm oil) plus thickenning agents such as soy lecithin to end up with similar consistency. That'd be included in the 28%.

In my country cacao refers to the cold pressed stuff where cocoa means the beans have been roasted. But it's different all over.