How are SPF vocations selected?

Hello all, I'm going to HTA on the 11th of feb and I have a few questions regarding it's vocations. I'm PES B1 and I have zero clue what vocations are available to me after POBC.

- What is the process like before knowing where you are posted to?

- Do I have to be interviewed or IPPT Bronze/Silver/Gold or be in perfect health? (Idk if this affects where I can go but I have eye colour deficiency)

- Are there any vocations that NSFs are more likely to go?

I'm not too sure if this is confidential, if it is, I'd love to know anything else related to the vocations or any stories from those who finished/currently doing their service. Much thanks!