Dimensions are divided into 4 types.

1. A shared dimension. A dimension which spills out into other dimensions. Various matter movement limitations might apply.
2. A fully open dimension. Matter can move in and out with no limitations.
3. A partially enclosed dimension. Matter can move in and out, however matter original to the dimension cannot leave, or original matter migration is greatly limited.
4. A fully enclosed dimension. No matter can go in or out.

Type 1 dimensions are common. Most of the time matter can move in and out with no limitations, but in some cases various matter limitations might apply. A typical scenario is a time shift, when a dimension is completely identical, but shifted into either past or future.

Dimensions which have no matter limitations and yet do not spill into other dimensions are classified as Type 2. These dimensions require the Normal Portal to be accessed.

Type 3 dimensions are either not common or more difficult to locate. They feature a one-way biased entrance, usually pointed inwards, so that entering the dimension is straightforward, while going back is difficult or impossible. The funnels of Type 3 dimensions also tend to have more extreme limitations aside from a one-way bias.

Type 4 dimensions cannot be studied directly, but they have been inferred from measuring interdimensional variance. They have also been probed via the Strange Portal.

A large number of theories have been proposed over the years in order to explain why different types exist. The most accurate model to date