"Designated emphasis" on a doctorate
Something like this is actually very common in some fields. Economics programs usually have a requirement called "Specialization Fields," "Major Fields," "Field Certification," or something like that (some example requirements). 

They usually consist of a couple courses in the same area, as well as often an additional field exam covering topics from those courses. Glancing over some requirements, most programs require you to finish ~2 or these as a requirement for getting your PhD, either equally or as a major/minor setup. As you might expect, almost all students do these in fields that they are interested in and will eventually do research in.

As for purpose, I believe that they are mostly used as a signaling and filtering device for the job market. If you look at any department's list of job market candidates, you can see that the specialization fields are very prominently displayed. I think the importance of these is probably linked to some of the idiosyncrasies of the econ job market, such as its degree of centralization, but that's just my speculation.