Points available for support can be used to buy allied forces from other Warhammer 40,000 Codex books. These can be Imperial forces, such as Space Marines or Imperial Agents, or allied forces such as Eldar. You will need to refer to copies of these books to choose your supporting forces.
Note that allied troops included in the army do not benefit from the Imperial Guard Command rule.
Space Marines
Space Marines are genetically enhanced warrior brothers whose devotion to the Emperor is absolute. Space Marine Chapters have their own spacecraft and can deploy rapidly to counter invasions or rebellion. Small contingents often arrive at battle-zones ahead of the ponderous Imperial armies, and subsequently find themselves fighting alongside the Imperial Guard.
You may include troops chosen from any of the Space Marines Codexes: Space Wolves, Ultramaries or Angels of Death.
Imperial Agents
Imperial Agents covers several quite separate and independent fighting forces. These include the forces of the Inquisition and Grey Knights, the Adeptus Arbites, the Ecclesiarchy including Adeptus Sororitas, the Adeptus Mechanicus and the Officio Assassinorum.
You may include troops chosen from the Imperial Agents Codexes, including: Codex: Sisters of Battle and Codex: Assassins.
Eldar
The Eldar are as likely to fight against the Imperium as they are to ally with its forces. However, in a hostile universe even the keenest of rivals must sometimes make common cause.
You may include troops chosen from the Eldar Codex with the exception of the Avatar. You may not include an Avatar as part of an allied force.
Squats
The Squats of the Homeworlds are a proud and independent race that evolved from mankind. They are divided into many rival Leagues. Although Squats and the Imperium have fought long wars in the past, the most powerful Leagues are now firmly allied with the Imperium.
You may include troops chosen from the Squats Codex.