Thanks for reply, Also I want to know, how to prevent leaving cached files in this folder mentioned before after picking files from PHPickerViewController ? I had tried several apps and found that 「Gmail」and iOS built-in app like 「Notes」won't leave cached files after picking. (Also verified that through backup file) And 「Microsoft Outlook」will leave cached files after picking. Is there a way to do that? Like removing cached after picking ? Thanks for your reading and looking forward the answers!

Yes, only for background session.