Originally, the Prima Categoria was the equivalent of the Serie A, until 1922 this was in fact the official name of the Italian top division.
A variable number of teams (generally 16 or 18) progressed to the national phase run by the FIGC itself.
When this amateur system did not match the interests of the growing richest clubs of the Northern industrialized part of the country, they left the FIGC to create their private super league, the Northern League, the ancestor of actual Lega Serie A.
After a year of talks, the clubs agreed to come back to the FIGC and the Prima Categoria was abolished and divided in two separate leagues, the Prima Divisione for the greatest teams and the Seconda Divisione for the others.
Established recently in 1959, is organized by regional committees of the FIGC, and therefore does not have a predefined structure.