Semi-sweet chocolate also goes by the name of “dark” chocolate, at least when it’s in candy bar form, though interestingly it often contains more sugar than even most milk chocolate. Semi-sweet chocolate is often 60% sugar by weight. The rest of its volume is made up of cocoa solids (about 15%) and cocoa butter (27-32%). Like bittersweet and unsweetened chocolate it has no milk solids in it, which I suppose is what makes it “dark” by some standards. Semi-sweet has fewer uses than bittersweet in my opinion, which I think is why you don’t see terribly much of it except as an inclusion in chocolate chip cookies, muffins, ice cream, that sort of thing. A lot of people just like to eat it, and really, who can blame them?
I should insert here once again that the definitions of bittersweet and semisweet tend to blur together in the US where, unlike Europe, there are no formal guidelines as to what constitutes either, and every chocolatier has its own set of standards. Your best bet for determining the relative darkness of a chocolate is to check the percentages on the packages which have become a de-facto standard in the industry the last twenty years or so.