I'm running Sync 2.4.4 (732) on Windows, not installed as a service.
I also have another instance on Linux which I'm using through the WebUI, but I'm not using Selective Sync on that (i'm using it as the always-on "master server" for all my synced folders).
I have a synced folder D:\Projects with Selective Sync enabled (on multiple Windows systems), and synced a different subset of subfolders (but having several in common) to these systems.
Now assume that I have a folder D:\Projects\ProjectA synced to all of them, and I create a new file D:\Projects\ProjectA\Testfile.txt on one system. It will get synced to the "master server" (without Selective Sync) just fine, but on the other Windows systems (with Selective Sync), only a D:\Projects\ProjectA\Testfile.txt.rsls file will be created.
As these folders contain source code where files are added regularly, and I want to be able to build from that subtree on any of the systems it's synced to, I have to right click the folder and select "sync to this PC" (or the translation of that in my OS language) every time I have added a file somewhere else, or the builds will fail due to missing files.