According to the Eleventh Doctor a pocket universe or pocket dimension was a "distorted echo of our own" universe. He indicated such realities occurred spontaneously but that they "never last[ed] for long". However, there are also examples of artificially created and longer lasting pocket universes. The Doctor also emphatically told Clara that a pocket universe was not a parallel universe.
The Galacti-Bank vault utilised dimensional transcendentalism. The contents of the vault were contained in a pocket universe which could only be accessed by using the correct combination, which changed four times per day. (AUDIO: The Selachian Gambit)
Hila Tacorien was once trapped in a pocket universe, and was only brought back to the normal universe thanks to the work of Emma Grayling, an empathic psychic who was able to, as the Doctor put it, act "like a lantern, shining across the dimensions, guiding her home".