Public Share & S-URL features can be used separately but they have the same configuration.
S-URL: File URLs generated by Amazon S3 services are very long and cause problems in some mail programs such as MS Outlook, and it is not easy to share. In order to use a shortened URL, you can use S-URL feature of S-Drive.
Public Share: You can share your files & folders to non-authenticated (non-Salesforce) users. You can set password, IP Range, Expire dates and so on. This feature is available in Lightning only and requires S-URL to be set up. Once S-URL is set up, you can use the Public Share action in the S-Drive component.
Setting up S-URL
There are two ways to set up S-URL and Public Share:
S-URL (Recommended): this setup uses a dedicated Community and a single dedicated community user. With this setup you can share links to non-authenticated users and allow them to open, download, upload and delete files as well as edit metadata. This can be controlled in the public link settings. Instructions for setting up S-URL using a Community
Legacy S-URL: this setup uses a Force.com Site with a Guest User license. This method will only allow for opening and downloading files using a public share link. You cannot have nonauthenticated users upload files, delete files, or edit metadata on files because of limitations of the Guest User license. You may need to buy Force.com Sites license to use this feature.
Instructions for setting up Legacy S-URL using a Force.com site
Enabling S-URL For Internal Download/Internal Open options
In S-Drive Configuration Micro Services Tab, choose whether to check “Enable S-URL for Internal Download” and “Enable S-URL for Internal Open.” These settings determine whether S-URL or Amazon AWS long URLs are used when a file is downloaded or opened by Salesforce users. If unchecked, S-URL will only be used for Public Share and Copy URL functionality. There is generally no reason to check these boxes. If they are checked and something changes with your S-URL configuration, you could have problems opening files internally.
Requiring password or expiration date
In S-Drive Configuration Micro Services tab, you may choose to require a password and/or an expiration date on all public share links. If checked, users will see those fields as required when creating a public share link and will not be able to copy the link until those fields are filled in and saved.