What is scale out deployment in reporting services?
Report server allows you to have multiple reporting services instances, while report server and report server temp DB shared among those multiple instances.
This is a better way to manage the performance aspect of the report. We can put a load balancer in the beginning of the report manager, which can pass on the traffic to those report servers, which is experiencing a less load.
You can easily add servers, and as we already configure the databases, it will by default pick the configured data source only.