Pocket Dimensions

Pocket Dimensions are the small pocket worlds that are created by walking through a newly placed Dimensional Door.

By default, this will create a large featureless room (bar the Dimensional Door you came through) with an interior size of 29 x 29 x 29, made out of 4 block thick walls of Fabric of Reality. After the 4 block thick walls is a one block wide shell of Fabric of Reality (214).

This wall can only be broken with a Block Breaker.

Beds
Beds can be placed in Pocket Dimensions, but similar to the Nether, you cannot sleep in them. If you try, the bed explodes, taking nearby blocks, including Fabric of Reality, with it.

Layers
Pocket Dimensions work on a system of layers.

Placing a Dimensional Door in the Overworld, Nether, End or Mystcraft Age will link to a layer 1 Pocket Dimension. Placing another Dimensional Door in a layer 1 Dimension will link to a layer 2 Dimension, and so on.

You can move up layers by placing Warp Doors. For example, placing a Warp Door in a layer 2 Dimension, will link to a layer 1 Dimension.

The coordinates used by a pocket dimension directly correspond to the coordinates of world that the first Dimensional Door of the chain was placed. So, going into a layer 1 Dimension (via a Dimensional Door), walking 3 blocks and then placing and walking through a Warp Door, will bring you back to the world you started in but 3 blocks away from the Dimensional Door you used to enter.

Dimensional Dungeons
Naturally occurring Doors Rifts and Rift Gateways will lead to a different type of Pocket Dimension, the Dimensional Dungeon.