>>508337313From what I understand, it's a priority system.
First, you try to reincarnate within your bloodline.
If that's not possible, within your race/people.
If that's not possible either, it's random.
It also depends of the souls being ready. If you die today you might not feel ready to come back until 300 years from now, but by then you won't be able to pick much.
There are hundreds of stories like that, such as a baby boy having memories that belongs to their great-grandfather, or someone having memories of living in a small Chinese village 400 years ago.