What is the difference between an M6 and M8 carabiner?

The M6 carabiner is supplied with the Australe 220 and Solar 200 sails and is suitable for use with standard long fasteners. The M8 carabiner is supplied with the Australe 340 and Acryl 300 sails and is suitable for reinforced long bindings. The M8 carabiner is not suitable for standard long fasteners.