The Cortex Global Filter allows marketers to prevent a segment of users from receiving email from any of the lifecycle stages, as well as Smart Blast emails. Those users can still receive email through Promo Blast sends.
How To Set Up a Global Filter
First, go to Audience > Segments and create a new segment. The output of this segment should only include the users that you want to receive Cortex emails.
Then go into your Cortex Settings, and select the segment as a Global Filter:
Once a filter is chosen as the global filter it will be removed from your list of filters. When it is replaced by a different filter (or removed as the filter) it will re-appear in the list.