Reverse Kalama rails...
Think of a surfboard rail. Then flip it over. No kidding.
Surfboard rail is "hard" on the bottom, and soft on top. Kalama does the opposite on foil boards.
Here's a 1-minute clip where you can see the rails in action.
The logic, I assume, is the foil board doesn't need a hard rail. The rail is not edging in the water like in surfing. You don't want it to grab.
The super soft rail on Kalama's board allows you to drive hard off a round house cutback. And importantly, if the board kisses the water, the rail does alter the path of the turn.
The Blue Planet board has a way-more-subdued version of this...
It's sort of a 50 / 50 rail, that has a way more pointy "peak" down the center than any 50 / 50 surf rail ever would. But it's all subtle.
I'm sure the goal is the same effect as the Kalama rail, which I think is for the rail to not grab the water and alter your path if it touches down on a turn.
I like the subtle Blue Planet rail and tail. It's not too crazy, therefore the board still acts like a SUP paddling around and setting up catching a wave, which is the most important part for most people getting into SUP foiling.
I haven't tried a board with the full blown Kalama rails. But I don't have the skills yet for those fully loaded carves like the top guys are doing where those extreme rails would really shine.