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

Estonia
Constitution
http://www.president.ee/en/republic-of-estonia/the-constitution/index.html
? 106.
Issues regarding the budget, taxation, financial obligations of the state, ratification and denunciation of international treaties, the declaration or termination of a state of emergency, or national defence shall not be submitted to a referendum.
? 162. Chapter I 'General Provisions' and Chapter XV 'Amendment of the Constitution' of the Constitution may be amended only by a referendum.