The Constitution is the governing document for the Union – everything the Union does must be compliant with the constitution.
The latest edition of the USI constitution came into force in April 2016, after amendment by USI Congress 2016.
Update: you may need to clear your cache (press F5) to flush an older link from your system to the older constitution. Some users have reported seeing a link to the older constitution here. This version verified on 23 April 2016.