The Deterrence Fleet is charged with protecting the Commonwealth from any threat, and is shrouded in secrecy. No-one, except from elements of ANA:Governance, knows exactly what it is, or even if it's a collection of spaceships at all.
It is revealed in 'The Evolutionary Void' that The Deterrence Fleet is not a 'Fleet', but an incredibly advanced device located on a planet orbiting Proxima Centuri which allows a person to essentially become as close to Post-Physical as possible whilst still being bound in the phsyical universe by a singularity point hidden within hyperspace layers. When Admiral Kazimir activates The Deterrence 'Fleet', he describes the sensation as being a soldier walking through an armoury as he arms himself with the tools required for his mission to stop the Ocein/Prime threat to the Greater Commonwealth.