Public file, only you (the person who added it) and your listed co-authors can view. While both public and private files, i believe you can only share if you hold rights for your publications (i.e, Publications under CC. licensing) for which you have paid APC.
In my recommendations, keep files private, and upon request by anyone you can always share that particular file.
The option gives you the possibility to limit public access to your manuscript, the public or private projects need to reupload due to such change, if required.