How do you identify which publication an Ad Bookings record is for?
You can define a self join that links ad bookings to a second table occurrence of the same table by ad type, booking ID, and whatever field identifies the publication to which this booking record belongs.
AdBookings::AdType = BookingCheck::AdType AND
AdBookings::PublicationID = BookingCheck::PublicationID AND
AdBookings::BookingID ≠ BookingCheck::BookingID
For ad types such as Front Cover, Not IsEmpty ( BookingCheck::BookingID ) means another record for the same publication has already been created for the front cover.
To create a "second table occurrence", click AdBookings in Manage | Database | Relationships to select it then click the button in the lower left corner with two green plus signs.
If "table occurrence" is a new term see this link for more on what they are and how you can use them in FileMaker: