The Structural limit as quoted in the AFM is 1 of a number of things which can limit the MTOW of any aircraft.
For large aircraft (EASA CS-25) there is a general philosophy that the loss of the most critical engine at any time throughout the flight should result in a safe outcome.
So when operating to those rules the MTOW is the lowest of all the limiting weights.
These could be related to the length of the runway or the climb capability on one engine or the ability to clear obstacles in the take off path (one engine) which may require higher climb capability or of course the certified structural MTOW.