In Scottish clans, septs are families that followed another family's chief. These smaller septs would then comprise, and be part of, the chief's larger clan. A sept might follow another chief if two families were linked through marriage; or, if a family lived on the land of a powerful laird, they would follow him whether they were related or not.
Today, sept lists are used by clan societies to recruit new members. Such lists date back to the 19th century, when clan societies and tartan manufacturers attempted to capitalise on the enthusiasm and interest for all things Scottish. Lists were drawn up that linked as many surnames as possible to a particular clan. In this way, individuals without a "clan name" could connect to a Scottish clan and thus feel "entitled" to use its tartan. Of course, in some cases, tartans have been created sometimes in more recent years, by members of some Septs.
In the case of the Scott Clan, Geddes, Laidlaw and Langlands are regarded as Septs of Clan Scott. However, some sept lists suggest that Geddes is a sept of cln Gordon as the Geddes name originated in Nairnshire. And "Balwearie" is also included in some Sept lists (but is more properly regarded as a "Cadet" rather than a Sept). "Buccleuch" is sometimes regarded as a "Sept" but that is the Dukedom associated with the Scotts themselves. However, as the name has also produced a number of miss-spellings (especially in North America) a page on Buccleuch has been included in this genealogy section.
Note that Napier is NOT a Sept of Clan Scott - though it is sometimes included in lists of Septs
In heraldry and history, a cadet branch consists of the male-line descendants of a patriarch’s's younger sons (cadets). Such off-spring were not expected (required?) to produce any offspring - though of course may did so. In some cases the cadet branch would become successful and perhaps own land and be awarded titles - laird or a knighthood or even higher rank. Such significant branches often became associated with a particular geographic location and became acknowlegded as (Surname) of "X". There are many younger sons in the Scott line who achieved such acknowledgement of their success - a selection of the Scotts who have been identified in this way are listed in the Cadet page of this site.