File/Attachment Sync takes files that are uploaded to Salesforce Files or Salesforce Attachments and syncs them with S-Drive for the selected object. Sync is processed based on a queue, so new items will appear in S-Drive in a few seconds/minutes.
Once you configure File/Attachment Sync with S-Drive, you can start using File/Attachment Sync features. See File/Attachment Sync to configure this feature.
Below is an example screenshot in Classic after user uploads a File to the Files section of the Accounts. It creates into the S-Drive. This feature works in Classic and Lightning.
You should also note that sync operation only happens once the file is uploaded to the Salesforce Files/Attachments section. Deletion or updates to the Salesforce File/Attachment do not delete or update the synced item in S-Drive section. Sync is triggered only with addition of a new Salesforce File/Attachment. Sync is only one way, so any changes to the S-Drive are not reflected to Salesforce File/Attachment. After sync'ing, those files/attachments are two distinct objects.
This process also does not sync the previously created Salesforce Files/Attachments before the sync configuration. You can sync those files with S-Drive navigating to "S-Drive Configuration - > Micro Services -> Sync Salesforce Files/Attachments with S-Drive -> Configure File Sync of Pre-existing Files" section.
A note about Permissions
File Sync and Attachment Sync use a callout to sync the file or attachment to S-Drive. So you must configure the outbound message user correctly:
Check Setup>outbound Messages for the following 3 items
Check the "user to send as" profile. It must have
Customize Application permission
Send Outbound Messages permission
Query All Files permission (which can only be applied using a permission set.)
Users who upload files that will be synced must also have profile permission for the Attachment Sync object.