Organisers can ask for details of their own: licence number, accommodation, meals, or proof such as insurance or a parent's consent. What is asked is up to the organiser of each competition.
Uploaded proof is stored in a non-public area. It can only be retrieved by the administration and by the organiser of that competition, and cannot be guessed from a link. More on this in the privacy policy.
If a competition asks nothing, you skip this step with “Continue”.