If an item on the MEL is marked with an (M), what does it indicate?

An item marked with an (M) on the Minimum Equipment List (MEL) indicates that maintenance procedures must be completed before flight. This signifies that the particular item is critical to the aircraft's airworthiness and safety, and cannot be deferred under any circumstances.

In essence, the (M) marking establishes that the item requires a maintenance action—either repair or replacement—prior to the aircraft being cleared for flight. This is crucial because allowing a flight to continue without addressing a maintenance item defined in this manner could compromise safety standards and regulatory compliance.

The other markings in the MEL serve different purposes, such as allowing flights to proceed under specific conditions or necessitating reference to the Approved Flight Manual, but the presence of the (M) specifically denotes that maintenance is mandatory prior to operation.