Pre-legislative referendum
Parliament passes a law calling the referendum on X but does not pass the details of X, so implementing a result would require further action E.g Brexit
Post-legislative referendum
Post-legislative: Parliament passes the laws to implement X, subject to a referendum on X, so if the referendum goes ahead it can be automatically implemented. E.g. the good Friday agreement
Initiative Referendum
Parliament passes a law allowing people to call referendums in certain circumstances, with the result being automatically implemented.
e.g Torbay and any Mayoral election