The booked status is similar to the replied status where the contact is taken out of the email sequence and will no longer receive email follow-ups from Interseller. They are brought into the "booked" state when you add them as an invitee to any calendar event in your Google Calendar or Outlook Calendar. The contact can also be brought into the "booked" status when they create an event on your calendar instead of being invited to an event on your calendar. The invitee's email address must match the email listed in the sequence in order for the "booked" status to appear. If the invitee uses a different email address to book an event on your calendar, it won't appear as "booked" on Interseller.
You can also set them as booked by clicking on the contact's status and marking them as "booked" to keep your stats up-to-date.
Note: They must be listed as an invitee or they won't be tracked properly. If you're using a service like Calendly, you may need to upgrade in order for it to add the invitee properly.