In case `rpc_public_addr` is not set, but autodiscovery is used, this allows filtering the list of automatically discovered IPs to a specific subnet. For example, if nodes should pick *their* IP inside a specific subnet, but you don't want to explicitly write the IP down (as it's dynamic, or you want to share configs across nodes), you can use this option.
These are the sources for the documentation but not the whole website. The website templates and other things are in garage_website, which uses this as a submodule.