The Union is the main authority in the Outer Spur. They require a quota of Rift Energy from the Rifters who mine the substance. They also maintain tugs, such as Union Tug Albatross, in order to transport the Rift Energy out of the Outer Spur. They also lend their name to a weapon in the Starhawk universe, called the Union Shotgun.
In the first level of the campaign, Cutter angrily refers to the sheriff as a "Union rent-a-cop", suggesting that the law enforcement is hired and/or run by the Union.
The Union maintains tugs in order to transport the Rift Energy from system to system. These tugs have an appearance of a train, in that there is a front, and container cars being carried behind it, further evoking the Old West theme of the game.
The transportation system is similar to the one found in Star Trek in that it "jumps" from system to system in or past (what is assumed to be) the speed of light.
Not much is known about the Union, whether it is a government, or a private megacorportation, and its only possible employee is the sheriff.
White Sands is required to fulfill a quota for the Union in Rift Energy, and there is seemingly nothing exchanged for the cargo, implying the Union may be a government, and if so, possibly socialist. Or, it may be a special type of tax imposed in order to maintain Union operations.