When creating a form, you can add File Upload fields, if you set an Upload Category, several of these options require an expiry date.
When an applicant completes a form, they will submit a document, and enter an expiry date.
It is possible that between the documents being uploaded, and your event date, the expiry date will pass.
There are several places that this is shown:
On the applicant's dashboard, it will show like this:
There is help on how to update an expired doc for applicants here.
Applicants will also receive a reminder email at several increments before the doc expires:
1 month
2 weeks
7 days
1 day
If the applicant has uploaded the same document to several forms, it will group the reminders together too.