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

Bahrain
BahrainConstitution
http://www.servat.unibe.ch/icl/ba00000_.html
Law by decree No 14 for 2002
Exercising Political Rights http://www.shura.bh/en/LegislativeResource/PoliticalLaw/Documents/Political%20Rights%20Law.pdf
Bahrain Constitution
Article 43
The King may conduct a popular referendum on important laws and issues connected with the interests of the State. The issue on which the referendum has been held is considered to have been agreed upon if approved by a majority of those who cast their votes. The result of the referendum shall be binding on all and effective from the date it is declared, and it shall be published in the Official Gazette.