Each Mafia family has a "core" group of about 100-200 "Made" guys, who are absolutely pure Sicilian. They are also called wise guys or good fellows.

To be "made" you have to go through a ceremony (hence the scene in Goodfellas where Tommy dresses in a silk suit!) You prick your finger with a needle, burn a paper religious image, and one or two others. An older made guy is designated a mentor.

The core of Made men is outnumbered by a large number of associates. Associates (or "connected guys") do not have to be Sicilian, and can't become Buttons, Capos, etc. Mafia families are made up of mainly connected guys, with only a tiny number of Made guys.

(It is sort of like the Spartan system of having a minority of Spartiates and a majority of Helots).