Suggestion: Please consider adding or allowing users to edit the schedule board and add Billing Type field which billable/ non-billable choice is available when booking a resource since one resource might be billable this month but non-billable next month (which project team can't control).

This selection would then be saved to the BookableResourceBooking or BookableResourceBookingHeader table,

together with other booking information such as: Resource, Start Date, End Date, Booking Method, etc.