Build audiences from identity matches

You launch a new product in one brand and want to email the people who already buy from your other store. Or you want to clear out duplicate records before a big send, and need the list of who to check first.
The enemy is a list of matches you can read one contact at a time but never act on as a group.
The audience builder has a new Identity match rule. Pick a possible duplicate in this brand, also a contact in another brand (any brand of your organization, or one you choose), a contact in at least N other brands, or merged with another contact in the last N days. Each rule reads Is or Is not, and the live preview counts the contacts who match. You can only pick brands you can open yourself, and matches never look outside your organization. The audience reads the matches again at every refresh. A fifth option, the same browser seen on another brand, is in a pilot and appears only where it is switched on.

Try it: open Reports, then Audiences, click New audience and add + Identity match.
Documentation