The Interstellar Alliance is made up of two separate political factions, the powerful Core Systems centered around Sol and made up of the founding worlds, and the far flung but more numerous colonies of the Allied Systems. Both the Core and Allied Systems are represented in the Alliance by their Senators.
The Alliance Senate is an administrative body of the Interstellar Alliance where senators for the Core Systems and the Allied Systems meet to vote on issues that affect the Alliance.
The Interstellar Alliance Central Embassy is located on Earth. From here they oversee the Interstellar Alliance's expansion initiative, which led to expansionary colonization from the older Core Systems into the newer Allied Systems. The Trade Leagues operate under its purview.