New Balance numbers refer to the different models of shoes. The main headline number (e.g. 623) designates the overall line of shoe. The last 2 digits provide more specific detail on factors like fit, cushioning or purpose: 06 to 19: Stability shoes like the 608 and 623, 20 to 29: Cushioned neutral shoes, 30 to 39: Lightweight/fast shoes, 40 to 49: Trail shoes, 50 to 59: Cleated shoes, 60 to 79: Workout shoes like the 619, 80 to 99: Lifestyle shoes. So in summary, the main number identifies the broader category while the last 2 digits get more specific on the exact style and build.