In Serbia, people from ERE were called Romeji, meaning Romans. Not Greeks, nor Byzantines, only Romeji.So, yes, Serbs, Bulgarians, Vlachs (from Great Wallachia in Thessaly) etc. were also Byzantines/Byzantine subjects, just like Greeks, Armenians etc. in different times. Of course, since Serbs and Bulgarians were non-Byzantines most of the time, while the Greeks were almost always Byzantines, many people consider "Byzantines" to be equal to "Greeks" (especially since the official language of the state was Greek).