Fast Backup doesn't actually record a database of what is on the Destination. It records a database of Source-now (as of run-X) for use next run-Y to compare the database to the actual Source state on run-Y (and can thus work out the differences = changes) without needing to scan the remote Destination*. It then updates the database to reflect state on run-Y, ready for run-Z in due course. And so on.
Ipso facto, it must be installed and used on the Source end (remote clients, in your case)
* 'remote' from the clients' point of view, that is. Bear in mind that on the first run as a Fast Backup it has no database (it is about to build one), thus it will scan both sides on that run, but not thereafter unless you make any settings or changes that force a Rescan
Ipso facto, it must be installed and used on the Source end (remote clients, in your case)
* 'remote' from the clients' point of view, that is. Bear in mind that on the first run as a Fast Backup it has no database (it is about to build one), thus it will scan both sides on that run, but not thereafter unless you make any settings or changes that force a Rescan