The Seal Realm is a unique, one-way dimension controlled by the Pure Souls. The dimension is said to be the only thing able to permanently seal the Tails Doll and prevent it from reviving if it is killed.
The Seal Realm is what is known as a "blank dimension." It is a vast, infinite void of absolute nothing but white blankness, accessible only by a portal opened by its master. There is no detail, no ground, no surroundings and normally no life (unless a living being is sealed inside). Only one member of the Pure Souls has control over the Seal Realm, being able to open and close the portal leading within at will.
If one is cast inside, unless they are released, they are never able to get out, being forever cursed to wander inside the blank void. The Seal Realm is said to have properties that still one's basic needs. If sealed, one cannot age, starve, tire, or even die. Being trapped within is not preferable, and is thus is kept as a last resort by its master.