Files with extension .rsls (Resilio Sync) are placeholder files created in folders with Selective Sync enabled or on a device in Connected mode. This is a pro feature and lets have access to shared data on demand
Placeholders only represent the files in the share, without the file content itself, as 0 byte files. It allows to save space and download the file only when it's actually necessary.
There are several points where you can do that:
1. When connecting to shared folder via link, switch the Selective toggle into 'ON' position (if you leave it OFF, the device will immediately start downloading all the content of the shared folder).
2. When the folder is already added. If your folder is already added to Sync and fully synchronized, but you'd like to select just a few files to keep syncing while turning the rest into RSLS files, simply turn Selective Sync ON:
3. While linking your devices. When you link your devices to one identity, you choose how each device will treat folders added on a linked device:
WORKING WITH RSL FILES IN FILE BROWSER (WINDOWS and OS X)
Turning placeholders into actual files.Working with subfolders
Go to the folder where you will see these placeholders. They do not contain actual data - only file names/filetypes. They show you what data can be fetched to your device on demand.
To fetch the actual files, simply double-click on a single file. Note that the single file click will open the file automatically if it takes less than a minute to download it.
Else, use the contextual menu option 'Sync to this device'. Multi select is supported.
This option also works when you want to Sync the whole subfolder: all files inside it will be downloaded.
Later, if source peer adds files in this subfolder, the files will be also automatically downloaded here.
If you're done with the file and want to revert them back to placeholders you can either use system's Delete (Move to Trash) or "Remove from this device" menu. File will be reverted to placeholder locally, and copies of file will be preserved on other peers.
If you have Read & Write access, deleting a placeholder removes it permanently from all peers. You can delete the files either through context menu "Remove from all devices" or delete the placeholder using system's Delete (Move to Trash) command.