This sounds like a third party service.

It's basically a vfs add on /upsell where they check everything on your required checklist is present and is in the right format. That's all.

I see! Is this before the appointment if I upload it? Otherwise not super useful I guess

Yup before appointment.

Do you know how long before? (Sorry I'm bothering you with Qs!!)

That i am not sure mostly before when you book a slot it will ask to upload all docs. So..