The United Planets Assembly actually was first formed in Year 12,919 under the Star Imperium under the name of the United Assembly of the Stellar Sectors. It from the beginning has been appointed by the assemblies of the Stellar sectors, which in turn were elected by the subsector assemblies.
It assumed it's present authority as the legislative body for much of the known universe and the United Planets when the Imperium became the United Planets in Year 13,100.
The legislation of the Assembly are subject to the veto of the Council of Twelve, for bills passed by the Assembly had to receive approval of at least 7 out of 12 Council members.
The United Planets Assembly has a total of 675 members representing 27 sectors, with each Stellar sector having 25 representatives.
The Monotheist Party was always the singles largest party, usually had between 40 and 60 percent of the Assembly seats, while the the Incarnate Party was always the second largest party in the Assembly with the Freethinkers generally in third place with 5 to 15 percent of the Assembly seats.