What is the .prefilled file for?

I've been analyzing the projects folder and I can't sort out what the .prefilled file is for. Any insight?

if you get a box for categories to choose during submission, this is the file where your choice is stored to not bug you again with the same query.

Thanks! For further clarification, in a project's options settings this is the setting I believe you are saying will affect this file: "If a form field can't be filled (like category)".

So if that is set to Skip, or Choose Random we won't get anything in the prefilled file. But if it is set to Ask User we will. Is that correct?