Which service is specific to the feature and required to be activated?
XSF: XMPP Standards Foundation (@xmpp@fosstodon.org) - Fosstodon
Configuring the Federated Server to Work with Vocera Platform
XMPP - Wikipedia
MP451
Interdomain Federation Guide for the IM and Presence Service, Release 12.5(1) - IM and Presence Service Configuration for XMPP Federation [Cisco Unified Communications Manager (CallManager)] - Cisco
Chat and Presence Federation Using Expressway Deployment Guide (X14.0.2) - XMPP Federation through Expressway [Cisco Expressway Series] - Cisco