There might be two guests staying in a room and you may need to send the invoice or confirmation to the second guest.
In a scenario like this, you will need to make the second the main guest of the room. Anyone whose name is entered first in the reservation is considered the main guest and by default receives the confirmation and invoice emails.
You can drag and drop the second name in the first spot (as shown below) and you will see after the spot changes the second guest name becomes green. This means now he has been marked as the main guest.