Wheel hardness on the A scale, e.g. 84A. Higher means harder, faster and louder; lower means softer and grippier.
Wheel hardness is the cheapest lever on how a skate feels — and the most frequently ignored. Roughly: 78–84A is soft, damps rough asphalt, grips well and wears faster; 85A and up rolls more easily and lasts longer but transmits every bump. For outdoor fitness skating on normal asphalt 82–84A is usually right; on smooth indoor floors 85A+. The second number is diameter in millimetres: bigger means faster but tippier and slower to accelerate. And an often-overlooked point — a bearing's ABEC rating tells you less about real speed than wheel hardness, cleanliness and lubrication do.