Requirements
A constitutional referendum can be called only when a constitutional law or constitutional amendment has been approved by both legislative Houses of the Parliament of Italy (the Italian Chamber of Deputies and Italian Senate) with a majority of less than two thirds in both or either Chamber, and only at the request of one fifth of the members of either House, or 500,000 electors or five regional councils.
In contrast to the popular referendums, the constitutional referendum is confirmatory. This means a "yes" vote means you want to retain the law, whereas voting "yes" in a popular referendum means you want to abolish the law.
In addition, a constitutional referendum is not subject to a quorum and is valid regardless of how many electors go to the polling station.
Read more about this topic: Referendums In Italy, Constitutional Referendum