What determines whether an SM analyst gets an MM PM vs MM snr. analyst seat?

Been seeing lot of moves from SMs to MMs at the senior analyst level.

Half the time the person gets a book, the other half joins as a snr analyst with promise of carve later. What determines whether the MM is comfortable giving them a book to start? Sure, "SM prestige" or risk management experience matters, but sometimes 2 people w. similar YoE from similar (or even the same) SMs will move to MM, and one will join as A and one as PM.

Is it historical PnL (to the extent one can explain their direct contribution)? Is it ability to convince BD/HR that you crushed it or had PM-type experience? Or is it that the people who joined as analysts had the ability to join as PM but wanted to learn risk model first