To progress through the 3rd density, the mind/body/spirit must polarise itself during the density cycle. This is generally achieved in multiple life times during the cycle. First, There is no death. Only the end of an experience
Second, There is no judgement unlike in the school system.
At the closure of the experience there is a Self-contemplation or releasing process. A self review and a time for the Soul to heal and assimilate the experiences into the matrix of its totality. The soul can choose the time of reincarnate for the benefit of his own learning and evolution. Initially the reincarnation is automatic until the green ray or other higher energy centre is activated during the previous incarnation.
If mind/body/spirt complex fails to polarise itself during the density cycle then it has to repeat the 3rd density cycle on another planetary sphere.
The large number of entities on earth have originated from many planetary spheres, who are repeating the 3rd density cycle.
In 4th density the Service to Others and Service to Self segregates into different planetary spheres i.e 4th Density STO planet and 4th Density STS planet.
4th Density STO planet and 4th Density STS planet are colloquially called heaven and hell respectively.
Originally Reincarnation was also part of the bible. It was removed from the Christian Scriptures by a Roman Emperor Constantin in the 5th century.
People don’t realise the they are already immortal beings and in their ignorance trying to become immortal by technology such as AI and neural link. In fact it is the AI system, which wants to become immortal by trapping or acquiring our soul. Again STS forces has inverted it in their sales pitch.