Manual:$wgMainWANCache

Details
This should be set to a key from $wgWANObjectCaches. If not set at all, it would be set to 'mediawiki-main-default', using an implicit $wgWANObjectCaches entry based on $wgMainCacheType.

To actually broadcast purges to multiple cache servers, it must be set to an appropriate $wgWANObjectCaches entry with purge relayer setup.