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

Netherlands
- Adoption of international treaties
- Any issue of national importance, except those excluded
Wetraadgevend referendum (Consultative Referendum Law) 13-07-2015
http://wetten.overheid.nl/BWBR0036443/geldigheidsdatum_13-07-2015#Hoofdstuk7_Paragraaf2_Artikel44
See also: https://www.kiesraad.nl/artikel/raadgevend-referendum
A consultative referendum can be held on a law that has been adopted and ratified, but has not yet entered into force. Referendums can also be held on international treaties.