Three different things with similar names
An initiative starts with citizens: proponents draft a measure and gather signatures to place it on a ballot directly. A referendum asks voters to approve or reject something a legislature has already done. A legislative referral is placed on the ballot by the legislature itself, which is how most constitutional amendments reach voters.
Roughly half the states allow citizen initiatives, and the mechanism does not exist at the federal level at all. There is no national ballot measure and no federal initiative process.
