Campaigns allow you to add several audiences to a single journey or broadcast. When adding multiple audiences, it is important to know that campaigns use OR logic for audiences.
This means that if you have two audiences on a single journey, that message will send to everyone who is in either audience.
For example, if you have a journey with two audiences on it: Guests who booked at least 7 days in advance and Reservations with at least one child on them, everyone in both audiences will receive the message. So, guests who booked 3 months in advance but have no children will receive the message and guests who booked the day before with one child will also receive the message.