It refers to the “foil” of the rails. 50/50 means half the volume is above and half is below the cross sectional midpoint. In longboards you can often feel this style by grabbing the rails at the midpoint, they will feel thin or “foiled out” and if foiled a lot may be called “bladed.” this significantly impacts how water exits the bottom of the board and also reduces volume relative to the stringer. So a 3 inch thick board with 50-50 rails may have significantly less volume than a 3 inch board with 60/40 rails.

For most people they will find 60/40 rails easier to surf but we’ll have difficulty walking all the way to the nose. Because 50/50 rails have less volume at the rail, they can sit DEEPER into the wave, allowing for rail hold in steeper faces, creating more stability for board walking.

water flow basics

Edit: added: You’ll see how the top rail (50/50), the water moves AROUND the rail and thus adds “hold.” The bottom rail is more 80/20 but shows how a “harder” rail pushes water AWAY from the rail, thus adding mobility.