How are alternates decided?

Edit – the title should read 'alternate upgrades' instead.

I'm based in the UK and have been selected as an alternate for JET. I was wondering how the selection process works. For example, if someone has already submitted all their documents (ICPC and COH etc) are they likely to be chosen over another alternate who is still waiting on these documents? I would really appreciate some clarity into how upgrades are decided, especially with placements due to come out soon.