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

Belize
Referendum Act, section 2
http://aceproject.org/ero-en/regions/americas/BZ/belize-referendum-act-2012
Section 2
(1) Without prejudice to any law which provides for a referendum to be held on any specific issue, the National Assembly may by resolution passed in that behalf declare that a certain issue or matter is of sufficient national importance that it should be submitted to the electors for their approval through a referendum.