They are equivalent to amendments to the Constitution of other legal systems (e.g., the United States or Ireland).
Depending on the results of the second vote, the constitutional law may then follow two different paths: If a constitutional referendum is requested, the bill must be approved by a majority of votes cast by the whole electorate to become law.
One is established by Article 139 of the Constitution itself: the form of government of Italy, a republic, cannot be amended.
That limit was introduced to protect the result of the institutional referendum in 1946 in which Italians voted to abolish the monarchy.
It is read to mean that the head of state office cannot be hereditary but also the principle of popular sovereignty is encapsulated.
A particular category of constitutional laws is the special statutes of the autonomous regions of Italy.