The first number is the measurement of the Firefighter’s index finger. The second part of the size, the letter, comes from the width of the Firefighter’s hand across the knuckles. Please see the below diagraph chart from NFPA 1971-2018, Protective Ensembles for Structural Firefighting and Proximity Fire Fighting, for how to size the Firefighter’s hand. If measurements are in between sizes it is recommended to trade up.
MK-2 and MK-2 ULTRA are a more tailored functional fit than other structural gloves so many users trade up one size, especially those with relatively thick/hands fingers. It is recommended to size generously.