Accessing the Soul Society

Shinigami enter and leave Soul Society by unlocking the Senkaimon using a Hell Butterfly. Pluses, on the other hand, enter Soul Society through the ritual of Soul Burial. Spirits can be born in Soul Society as much as in the living world, with the shinigami acting to balance the souls.

Intruders can pass through the Senkaimon in the same manner, but only in soul form. This allows spiritual beings, such as Ichigo in his Shinigami form, to enter without permission. Humans can also use such a gate through the use of Reishihenkanki (霊子変換機; lit. spirit exchangers), which convert solid matter into Reishi's. While this method uses the same gate, the lack of a Hell Butterfly forces intruders to pass through the Dangai, a bordering dimension between the human world and Soul Society (not to be confused with Hueco Mundo). This is much more dangerous to do, as souls that cannot pass through before the gate to Soul Society closes are trapped there forever.

Finally, Hollows can also directly rip a hole to Soul Society from Hueco Mundo, or in the case of Arrancar, using their ability, Garganta.

When people die they are assigned a number to a part of Soul Society according to when they died. Spirits can be born in soul society as much as in the living world with the shinigami acting to balance the souls.

Senkiamon
The Senkaimon (穿界門; world penetration gate) is the dimestional gateway which Shinigami use to come and leave Soul Society by unlocking it. When guided by a Hell Butterfly, the Senkaimon takes the form of a waiting room entered through a sliding door. Humans can also use such a gate through the use of Reishihenkanki (霊子変換機; spirit exchangers), which convert solid matter into spirit particles. A variant on this gate using four marked points can allow entire towns to be moved into Soul Society, as is done to Karakura Town to prevent Sōsuke Aizen from destroying it.

Intruders can pass through the senkaimon in the same manner. While this method uses the same gate, the lack of a Hell Butterfly forces intruders to pass through the Dangai (断界; prohibiting world),. This is much more dangerous to do, as souls that cannot pass through before the gate to Soul Society closes are trapped there forever. It is possible for Seireitei to deactivate these mechanisms, allowing Shinigami or humans to traverse the dangai safely. While the Senkaimon are run by the Kidō Corps, during the Shūsuke Amagai arc it is shown that there are private Senkaimon own by rich families.

Hell Butterflys
Hell Butterfly (地獄蝶, jigokuchō) are completely black butterflies that serve various purposes. They guide the Shinigami through the Senkaimon when travelling between Soul Society and the Human World, thereby saving them from the inconvenience of passing through Dangai. Hell Butterflies are also used as messengers between Shinigami, often carrying important orders. It is implied that Hell Butterflies are bred and taken care of by Shinigami in Seireitei, the latter task being carried out usually by new and inexperienced members of the Gotei 13. Hell Butterflies, like the Shinigami themselves, are only visible to spiritually aware beings