There are two possible ways to filter content by teams in feeds.
The teams assigned to a feed
By default, all sources in a team-specific feed will only display content from the teams assigned to the feed. This means that in the feed in the picture below, only Achievements and Battles from Oslo will show up in the feed.
Custom source filters
If we press Set custom team filter for the Achievements source, though, we can set the specific teams we want Achievements from in this feed:
The result is that the feed will include Achievements from the teams Oslo and Berlin.