The changes, which take Italy from having no squad size rules to being in line with the standard used in UEFA competitions, limit Serie A clubs to 25 over-21 players. Of those, no more than 17 can be non "home grown" players, meaning players who spent at least three years between the ages of 15 and 21 as part of an Italian club, be it in their academy or first team squad, or even away on loan so long as an Italian club still holds the player's contract.
Adding a further layer of restriction is that, of the eight roster slots held for players who trained in Italy, four of those are reserved for "club trained" players, meaning players who spent at least three years in that younger age range with the club that currently holds their registration. While most Italian teams won't have any issue with the first home grown restriction, the club trained player requirement will likely trip up many clubs.
Players who are under-21's (not yet 21 at the start of the calendar year in which the season began) are unaffected by these changes and can be used freely.
Despite the difficulties many clubs will face in meeting these requirements, all 20 Serie A clubs have agreed to immediately restrict their squads to follow these rules*. The change was put in to place not just to bring Italian squad rules more in line with UEFA's, but also to help aid financial stability for clubs by reducing bloated squads while promoting the strengthening and better usage of Italian academies.