The ideas of Bulgarian nationalism grew up in significance, following the Congress of Berlin which took back the regions of Macedonia and Southern Thrace, returning them under the control of the Ottoman Empire. Also an autonomous Ottoman province, called Eastern Rumelia was created in Northern Thrace. As a consequence, the Bulgarian nationalist movement proclaimed as its aim the inclusion of most of Macedonia and Thrace under Greater Bulgaria. Eastern Rumelia was annexed to Bulgaria in 1885 through bloodless revolution. During the early 1890s, two pro-Bulgarian revolutionary organizations active in Macedonia and Southern Thrace were founded: the Bulgarian Macedonian-Adrianople Revolutionary Committees and the Supreme Macedonian-Adrianople Committee. The Macedonian Slavs then, were regarded and self-identified predominantly as Macedonian Bulgarians. In 1903 they participated together with the Thracian Bulgarians in the unsuccessful Ilinden-Preobrazhenie Uprising against the Ottomans in Macedonia and the Adrianople Vilajet. That was followed by series of conflicts between Greeks and Bulgarians into both regions. The tension were result of the  different concepts of nationality. The Slavic villages became divided into followers of the Bulgarian national movement and so-called grecomans. The Young Turk Revolution of 1908 restored the Ottoman Parliament, which had been suspended by the Sultan in 1878. After the Revolution armed factions laid down their arms and joined the legal struggle. The Bulgarians founded the Peoples' Federative Party  and the Union of the Bulgarian Constitutional Clubs and participated in Ottoman elections. Soon, the Young Turks turned increasingly Ottomanist and sought to suppress the national aspirations of the various minorities in Macedonia and Thrace.
Question: The Bulgarians and the Greeks had a series of conflicts after the unsuccessful Ilinden-Preobrazhenie Uprising against the Ottomans in Macedonia mostly due to their differing concepts of which one, the Peoples' Federative Party or nationality?
Answer: nationality