Tribal affiliations were especially common in earlier generations, while connections to towns and regions became increasingly prominent in later biographical usage. One narrator may correctly bear both a tribal and a geographical nisba.
A geographical label may describe birth, settlement, residence or another genuine association. The presence of two labels is not enough to conclude that two narrators are involved.
Research example: one chain calls a man al-Baṣrī while another gives his tribal name. Matching teachers, pupils and dates may establish that both forms belong to the same individual, though the full evidence still needs examination.