Short answers, pulled from the story.
The word clan entered the English language in 1406 as a label for the social organization of the Scottish Highlands. Its roots lie in the Gaelic word for children, offspring, and progeny. This linguistic origin reveals that these groups were living families bound by blood or the powerful fiction of shared ancestry.
Rwanda maintained a highly structured system of twenty clans divided into subclans. Nkore operated with only four clans. These groups functioned as the primary unit of social organization and dictated social standing, political power, and the division of labor.
Scottish clans became unique in their elaborate systems of tartans and insignias, while the Irish model focused on the chief and his extended family. In Ireland, unrelated clients could assume the name of the leading lineage to show allegiance yet remained ineligible to be elected chief. The Irish clans existed within a lineage-based society that was eventually dismantled by the Tudor Conquest.
The clans of the Chickasaw and Choctaw tribes in the Southeast United States shared fox and bear clans. These groups felt a kinship reaching beyond their respective tribes and demonstrated how these groups could transcend tribal boundaries to form broader alliances. This flexibility allowed clans to survive the collapse of larger political structures.
The Polish clans differ from most others as they are a collection of families who bear the same coat of arms rather than claiming common descent. This creates a unique system of heraldic kinship. Similar usage applies to specific groups of various cultures and nationalities involved in organized crime where the clan structure provides a framework for loyalty and protection.