More information on the use of population registers can be found in On the Move: Migration in an Italian Community, 1865-1921, published in Social Science History, authors: David I. Kertzer and Dennis Hogan.
"Each administratively defined houehold and each individual were to be continuously followed and their changing characteristics recorded. In the case of Casalecchio there was both a system of household folders and a system of individual registration cards with cross-reference to the successive household folders in which the individfual could be found through his or her time in the commune. Each resident of the commune thus had his or her own data card in the register, as well as appearing as an entry in the household folder in which he or she lived.
As part of the Italian population register system there were also separate immigration and emigration registers. Each person or family group who entered the commune had to register with the communal authorities, at which time a listing of the individuals was made and transcribed into a new population register entry. The inividual or family group was also listed alphabetically in a year-by-year index of immigrants. When a person emigrated, the commune of immigration was obligated to make a copy of the immigration certificate and send it to the commune from which the individual emigrated. These certificates formed the emigration register of that comune, and the information was used to provide a dated closure for the individual's population register entry. An alphabetically-ordered emigration index was also compiled."
This gives us a more intimate look into the civil process regarding population registers, emigration, and immigration that no book on Italian research provides. It pays to look beyond the standard references when you are trying to more fully understand your ancestors lives and the evidence you find within the records.