Apparently the first one or two digits indicate the 'quality' of design materials, higher number being 'better'. The last two digits indicate the 'performance series' for which the shoe is intended.
Copy/pasted info regarding the last two digits -
60: stability (860-1260)
70: mild stability (870)
80: neutral (880-1080)
90: lightweight, modern (890)
00: spikes and racing flats
40: Optimal Control
Supported by medical and podiatric experts, shoes in this category provide maximum control, stability, cushioning and support for biomechanical needs such as pronation and low arches.
50: Fitness Running
For training on roads or for indoor workouts, the 50 Series offers the combination of visual attitude and innovation with the responsiveness and power athletes need.
Designs that offer industry-leading stability to reduce pronation while also providing unparalleled cushioning and comfort.
70: Light Stability
70 Series shoes offer the perfect combination of stability and speed, all in a lighter, sleek profile designed for runners who train at a faster pace.
For high-mileage runners who require light shoes and the protection of superior cushioning.
For faster runners who want every advantage, including a superior ride and fit. The choice trainers for professional and non-professional speed and distance runners.
Racing flats and spiked shoes that define high performance. Competition-level footwear for athletes who want to maximize performance, set records and win gold.