1.) If you are notified every time there is a new post in the channel
2.) If you are notified every time the channel is mentioned
This improves the previous experience of having to follow channels in order to be notified.
It's not in the What's New section of Microsoft Teams app and doesn't appear to be in docs.com yet. However, it has been brought up on social (twitter) by several members of the community including @adam deltinger and @Vesa Nopanen