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

Spanish Constitution 1978 http://www.tribunalconstitucional.es/es/constitucion/Paginas/ConstitucionIngles.aspx

Mandatory referendums are specified in the SpanishConstitution one by one in a way that no open clauses or a list of excludedissues is ever used in the Constitution as regards mandatory Referendums. Inthe case of Referendums consultivos (optional) the Spanish Constitution 1978 inits article 92 establishes, without making reference to any specific exclusion,that ?Political decisions of special importance may be submitted to all citizensin a consultative referendum. [?]

N.B. (This so called ?Referendum consultivo ? is neithermandatory nor binding)