sept is an old celtic-gaelic word for amily ..and connotes a subdivision of a clan. When a man was permitted to start afmiy - he may have also bee granted a right to use another name - his or a variation of the Clan's spelling ... and sometimes they just devlothat way and were recognized; So also with families which joined Clans.|
MacNeil is the form clan ame ..McNeill, and MacNeill are 'septs'.
MacGowan's in some cases, the ubiquitous Scottish 'smith' ... soemtimes became 'sept' of Stirling, and MacPherson.
McCormack . because of their anciet Cormac connection became a 'sept' at times of Clan Buchanan (the censor of the realm - a stewart forerunner).