60/40 rails describe how the foam is distributed around the rail apex (the most-side-out point of the rail in cross-section). 60% of the foam volume sits below the apex, 40% sits above. The fuller bottom side gives the rail something to bite into when you load it; the leaner top side lets water release off the deck cleanly.
What it does on a surfboard
60/40 is the most common modern shortboard rail because it splits the difference between the two extremes. Fuller rails (e.g. 70/30 boxy) have more foam below the apex and feel more forgiving — they hold longer through turns but resist quick rail-to-rail shifts. Thinner rails (e.g. 50/50 pinched) have foam balanced equally above and below — they release water faster but feel touchy and unstable to less-experienced surfers.
60/40 in the mid section gives you grip when you want it and release when you don't — a balanced, predictable rail feel that works across a wide range of surfers and conditions.
What to look for
Most modern HPSBs run 60/40 rails through the middle of the board and transition to hard rails in the last 12 inches (the back third). On the model page, look for "60/40 rails" or "60/40 mid-rails" — it's the modern shortboard default. Lundquist boards run 60/40 mid + hard back across the line.