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).

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