Typical scenario is the following: a user puts a file into a TSS share. Shortly after the user decides to free up space locally and revert the file into a placeholder, not ensuring that it is uploaded to another Agent, thus losing the only copy of the file.
The new functionality is implemented to prevent dataloss of the single copy of file across all Agents. For it to work, at least one priority_peers must be configured in the Profile (see here for other use cases with priority_peers). The Agent will verify that the file is uploaded to the priority peers and only after that the file will be dehydrated. If a priority peer is not configured, the file will be dehydrated.
The warning appears in the Agent UI. Starting with Resilio Connect v3.7.0, it's additionally reported to the Admin on the Management Console. The file will be dehydrated automatically once it's uploaded, and the user cannot cancel it. If the user needs the file back, redownload it again from the main server.