Right now, when you use Google Assistant's Broadcast feature, it's an all-or-nothing affair. It will broadcast your message to all Google Assistant-supported speakers your account is linked to. But Google seems to be testing a way that it'll just broadcast to a specified room.
9to5Google reports that this seems to be an A/B test of the feature. Users will need to say something like, "Hey Google, broadcast a message to living room, the food is ready." And then Google will broadcast this message to the speakers located in the living room. It'll be handy for things like calling your kids down for dinner. But since it's being tested, the feature isn't available widely yet. Here's to hoping Google rolls this one out soon.