Support for the Slack Real Time Messaging (RTM) websocket API
The RTM API allows integrations to connect to slack from behind a firewall, seemingly connecting much like a normal desktop client might, from any computer, not just an externally visible server.
It's a very important enterprise feature for those wanting to integrate Mattermost with their internal applications.
Further discussion on this can be found here:
And this slack FAQ describes the differences between the Events and RTM API's:
This is the Slack RTM API spec: