Building on the "Theme" system, medieval writers used the term "Bulgarian" as a broad catch-all for the Slavic-speaking population under the jurisdiction of the Archbishopric of Ohrid.

1. The Geographic Confusion

The Archbishopric of Ohrid was titled "of All Bulgaria." However, "All Bulgaria" referred strictly to the borders of the Theme of Bulgaria (Macedonia, Serbia, Montenegro), which did not include the territory of modern-day Bulgaria (then Paristrion/Thrace).

1. Historical Citations on Serbs and Montenegrins as "Bulgars":

Laonicus Chalcondyles: "The Triballians [medieval term for Serbs/Rashka] are now called Bulgarians." (P.G., vol. 159, col. 26)

George Cedrenus: Refers to the Montenegrin Prince Konstantin Bodin as "Tsar of the Bulgarians" and his army of Montenegrins and Kosovars as "Bulgarians" because they were residents of the Theme. (Georgius Cedrenus II, Bonnae, 1839, pp. 714-719)

1. The Slavic Synonym

For Byzantine writers like Theophylact of Ohrid (a Greek from Euboea), "Bulgarian" was a synonym for the Slavic language. He even refers to himself as a "Bulgarian" in a geographic sense because he resided in the Theme.

Theophylact: "St. Clement handed down to us Bulgarians everything concerning the church." (Large Life of St. Clement, Moscow Manuscript No. 818)

1. The Evidence of Ethnic Macedonian Identity

Despite the official Byzantine labels, people still identified ethnically. A 13th-century record by Demetrios Chomatenos identifies a man named Ivan Ierekar from the village of Vlasto (Velestovo) specifically as:

"Macedonian by birth" (по род Македонец)

Source: J. Pitra, Analecta sacra et classica, t. VI, col. 315.

Conclusion: The term "Bulgarian" in the Middle Ages was often a bureaucratic or linguistic label imposed by Byzantium, much like "Yugoslav" was in the 20th century. It didn't replace the underlying ethnic identity of the people.

So you really want to go into history

One of the biggest misconceptions in Balkan history is that Byzantine sources from the 10th century onward refer to "Bulgarians" in Macedonia in an ethnic sense. If you look at the primary sources, the reality is purely administrative, not ethnic.

1. The Administrative Shift (972 AD)

After the fall of the first Bulgarian Kingdom in 972, Byzantium (Romania) reorganized the Balkans into Themes (military-administrative districts). To assimilate diverse groups, they renamed territories:

Theme Bulgaria (Voulgaria): Included today’s Macedonia, Serbia, and Montenegro. Consequently, all inhabitants (Macedonians and Serbs alike) were called "Bulgarians" by Byzantine officials—simply meaning "residents of the Theme Bulgaria."

Theme Macedonia: Shifted geographically to Eastern Thrace (around Adrianople/Edirne). People living there (mostly Greeks, Armenians, and Thracians) were called "Macedonians" in records, regardless of their actual roots.

Actual Bulgaria: Renamed Theme Paristrion or Moesia. Its inhabitants were called "Paristrions" or "Moesians," not Bulgarians.

1. The Case of Basil II "The Bulgar-Slayer"

The famous Emperor Basil II is called "The Macedonian," but he was ethnically Armenian/Slavic. He carries the name because he was born in the Theme of Macedonia (Thrace). Conversely, he was called "Bulgar-Slayer" not because he killed ethnic Bulgars, but because he defeated the inhabitants of the Theme of Bulgaria (Samuil’s state).

Source: “Basil I.” Encyclopædia Britannica. 2005 / Abstracts from the International Conference ARMENIAN CONSTANTINOPLE, UCLA, 2001.

1. If Macedonians were Bulgarians, then Bulgarians weren't!

Following the logic of modern Bulgarian propaganda: if Macedonians are "Bulgars" because they lived in Theme Bulgaria, then today’s Bulgarians are actually "Macedonians" because their territory was called Theme Macedonia or Paristrion at the time.