Trepanations can be divided into three classes: 1) Those performed on living subjects as a therapeutic method. 2) Those performed on a corpse for "magic" properties. 3) Incomplete trepanations that are performed on living subjects without fully removing a piece of the skull, perhaps only scraping off portions of the skull. The last ones are those signified as symbolic trepanations and are supposed to be performed for cultical reasons.
Several examples of this kind of trepanation are known from Hungary (according to Berecki, Molnár, Marcsik, Pálfi: Rare Types of Trephinations from Hungary Shed New Light on Possible Cross-cultural Connections in the Carpathian Basin in International Journal of Osteantrohopology, it was fairly common there in the 9th to the 11th century, cf. also this blog).
The term symbolic trepanation was introduced by the Hungarian L. Bartucz in 1950, and Yordanov himself has studied the phenomenon on finds from the West coast of the Black Sea (Jordanov, Dimitrova, and Nikolov: Symbolic Trepanations of Skulls from the Middle Ages (IXth - Xth Century) in Bulgaria in Acta Neurochirurgica Vol 92 (1988), issue 1-4, pp. 15-18).