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benjl said..
That's really interesting colas- so hypothetically if you wanted your board to be able to turn more easily from the middle of the as opposed to the tail of the board, you'd put hard rails quite far forward? But it wouldn't hold in a steep face as well?
If by hard we mean a sharp edge at the junction with the hull, it wont be important to provide hold.
And for turning from the middle, other factors are more important: rocker, width, rail volume distribution (thinner rails in the tail), outline curve...
For the question of the hold in steep faces, See this picture:

The right rail enters the water like an airplane wing, and the water wraps around the rail. Curving the water trajectory requires a force that pulls the board into the wave face and provide hold. The more it "bends" the water flow around it, the more the rail provides hold. You want the rail to "suck" water into going above and around it on entry, not cutting it. But as the rail edge, if present, is underneath it doesn't matter there: what is critical is the shape of the upper part of the rail, and the position of the rail main width (high/low)
In the following picture, you can see what you want to avoid: the water separates early from the rail and is not wrapped around, providing less hold than a proper rail:

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You can see however how the water cleanly separates from the board on the right rail above and the left rail below: this is where the rail edge helps separating and reducing drag:

Rail design is complex however. You want a gentle curve in it for the water to follow it without separating, but without being too round, in order to reduce volume so that it can penetrate deeper in fast waves. But on slow waves, you want the rail to not penetrate too much to reduce drag. Basically, you are better off telling your shaper what kind of waves you will be surfing, and what you want to do on them rather than trying to tell him how to design the rails.