Issues in relation to which referendums are optional at the national level

Croatia
Constitution of Croatia 2010
Article 87
The Croatian Parliament maycall a referendum on proposals to amend the Constitution, a bill or any suchother issue as may fall within its purview.
The President of the Republicmay, at the proposal of the Government and with the countersignature of thePrime Minister, call a referendum on a proposal to amend the Constitution orany such other issue as he/she may deem to be of importance to the independence,integrity and existence of the Republic of Croatia.
The Croatian Parliament shallcall referenda on the issues specified in paragraphs (1) and (2) of thisArticle in accordance with law, when so requested by ten percent of the total electorate of theRepublic of Croatia.
At such referenda, decisionsshall be made by a majority of voters taking part therein.
Decisions made at referendashall be binding.
A law shall be adopted on anysuch referendum. Such law may also stipulate the conditions for holding aconsultative referendum.