Allowlist: Explicitly list instances to connect to. BlockList: Explicitly list instances to not connect to. Federation is open to all other instances. Open: Federate with all potential instances.
Federation is enabled by default. You can add allowed and blocked instances, by adding a comma-delimited list in your instance admin panel. IE to only federate with these instances, add: enterprise.lemmy.ml,lemmy.ml to the allowed instances section.