Hi Khairul
The optimum capacity is the value that specifies the number of attendees best suited to the size of the room. Therefor the maximum number only use for the essential booking.
In other words, Determining booking priority: The optimum capacity is used to determine the values for normal, essential, and waiting list bookings. Bookings are automatically assigned normal priority until the optimum capacity of the business event has been reached. Once the optimum number of attendees has been booked, only bookings with essential priority are accepted up to maximum capacity.
Regards,
Shirin