Pocket reality manipulation is a power which revolves around manipulating and creating pocket realities.
There are at least two types for this ability:
Type 1: Altering the internal structure of the pocket reality - usually it means the user is able to control aspects of his/her personal pocket reality, like manipulating time, or warping reality. Most of the time, it's limited to the boundaries of the pocket reality.
Type 2: Manipulating a pocket reality as the unit - basically controlling the pocket reality without being limited to boundaries, allowing user to shift, merge, cut and destroy pocket realities.
Travel between realities is a sub-ability of it.
It should be noted that even if one possesses the first type, it doesn't mean he can't use the second one.
Very often various characters can achieve their full power only in their personal pocket realities, e.g. Hell-Lords such as Mephisto from Marvel Comics are greatly weakened outside their pocket realities.