Sept is an Irish word meaning family. It is used frequently prior to the mid-19th c. but before that time nobody considered that there was or should be a list of family names related to the Clan or, more correctly, the Clan chief. The attempt to formalize a list of family names associated with Clans is most likely the result of a desire by early Scottish tartan merchants to widen their customer base. Fact is, when the Clans were in their heyday, everyone knew who they were and what their association was with their particular Clan.

Many Clans, thru genealogical work, have established lists of "related family names" (a much better term) historically associated with the Clan. Much of this comes from public records although tradition is also important. Sir Crispin Agnew, an officer of the Lyon Court, wrote a very interesting piece on the definition of a "sept" back in 2001. It is available online by googling his name. I think it lives on the Internet in Electric Scotland's web site.

I would not disregard the association of Clans with families bearing surnames other than that of the Chief. There is ample evidence and historic precedent for the connection between many Clans and the surnames on their "sept lists".