It's just a length of profiled rubber channel that you bend to the shape you want it to follow and stick it with the attached 3M tape. The idea is that you mount it on the roof and A pillar and follow the shape of the door as in Peatey's pic above, and then bend the front down so that water runs down and drips onto the front wing instead of inside the driver's door when you open it. Simple but effective.
I have C channel awning rail on both sides and it doesn't help in the slightest. If you wanted to put it above the sliding door you would probably need a second length so you could channel the water forward and then down onto the wing as before. Saying that, I've never had a problem of water dripping into the sliding door when it's opened, I think it's just a design flaw of the front gutters (or absence)