This may either be voluntarily or, as is the case in many countries for a constitutional amendment, as a mandatory part of the procedure for passing a law.
These referrals, depending on the location, can either amend a constitution or enact a change in statute.
In a legislative referral, they only approve or reject laws which their legislature votes to place before them.
Bond issues are a common type of legislatively referred ballot measure.
Delaware is the only state in which the people do not have the power to vote on constitutional amendments.