Final answer:
The 'M6' in a metric fastener designation indicates a nominal diameter of 6 millimeters for the fastener.
Step-by-step explanation:
A metric fastener designation of M6 means that the fastener has a nominal diameter of 6 millimeters. When you see an M followed by a number in this designation, it refers to the metric measurement of the screw or bolt diameter. It's a standard notation used to specify fasteners and is critical in ensuring the correct size is used for specific applications, ensuring a proper fit.