The BSE Life Cycle
As many of you should be aware of by now. When you die in the game, you're race will change. There is a distinct pattern that BSE Life and Death cycle follows, it is the same one that has been established in the Manga & Anime.
The following image should illustrate how the cycle works in this game.
Depending on the race you start out as & what HRA (hidden race attribute) you've selected will determine how your cycle proceeds.
For example: If you chose to be a Shinigami with the HRA of Shinigami, you'll begin the game in Rukongai as a Spirit.. You'll have to go through some training to become a Shinigami.. If something happens to you once you're a Shinigami and you die (be it from an NPC or PVP attack) your race will subsequently change. Following the chart you would be reborn as a human. From there you would progress playing as a human, until you died and became a spirit again.. from there you'd need to be soul buried to reach Rukongai and become a Shinigami again. Otherwise if a Hollow devours you as a Spirit you'll end up in Hueco Mundo as a Hollow.
Humans can progress to either Bount or Quincy depending on the HRA chosen, all 3 of those races (Bount, Human, Quincy) when they die will become Spirits.
Spirits have 2 routes that can befall them; either they're soul buried and end up becoming Shinigami, or they are devoured and end up as Hollows. The only sidebar to this is if as a Spirit you perform seppuku (suicide) doing so will turn you into a Hollow. (it's the equivalent of removing your soul chain forcefully)
Only those who choose the Shinigami HRA will be able to become Vaizards. Just as only those who choose the Hollow HRA can become Arrancars.
Hollows and Arrancars upon death are materialized as Spirits, where Vaizards and Shinigami are reborn as Humans.