Universities generally have a set of subjects open for doctoral studies. In my own language we call them "forskarutbildningsämnen", which translates as "doctoral education subjects". Now I'm wondering what would be the most common/idiomatic equivalent in BrE – in translations from my own language to English the most common by far is "third-cycle subject", but I have come to understand that this is not widely understood by people in general? Among the alternatives I have thought of myself (PhD subject, doctoral subject, doctoral education subject, and research training subject), it seems that the most common is "PhD subject", followed by "doctoral subject" (using Google Ngram), but perhaps there is another word that I just haven't thought of that is more common? Also, if PhD subject is, indeed, the alternative most commonly used, I'm wondering whether it can still be used about all doctoral education – not only about the kind that leads to a degree of doctor of philosophy?
Note that by "subject" I mean the overall subjects/subject areas within which doctoral students can conduct their studies. For instance, English, comparative literature, physics and so on. So, not the specific topics that a certain doctoral student can write their thesis on.