An Ultraspace is a kind of shadow universe attached to a Hyperspace.
Ultraspace can be considered to be the 'Hyperspace of Hyperspace'- co-ordinates in Ultraspace correspond to those in Hyperspace but are a million times closer together again. An Ultradrive is required to access Ultraspace, and only intergalactic or above civilisations seem to have developed these sophisticated technologies.
Matter which is not held there by an Ultradrive will instantaneously drop down into Hyperspace, where if it has no Hyperdrive it will drop down into real space.
An object travelling through Ultraspace can enter an area of Hyperspace denial, but when it drops down into Hyperspace it will be unable to move within that area- only able to drop down from there into real space or head back up to Ultraspace.
Objects in Ultraspace cannot detect objects in Hyperspace or real space and vice versa, unless the threshold power is given to the Ultradrive in which case the object will exist both in Hyperspace and Ultraspace.
In turn, Ultraspace has its own 'Hyperspace', the Uberspace.