That option isn't specific to that node, because it's a rather "common" thing shared by other nodes.
Final position = use coordinates of final position = coordinates of final displaced surfaces, so the "state" of the terrain before it goes into the planet node.
Texture coordinates = use texture coordinates of current position in the network, so if you haven't computed a terrain yet it will use the texture coordinates of the undisplaced planet surface.
After your first compute terrain it will use the texture coordinates provided by that compute terrain node.
If you have displaced your surface then the compute terrain will update the texture coordinates for the displaced surface.
A warp shader can only warp texture space so you need to make sure the SSS outputs it's shape in texture space, so that's why you don't use "final position".
Since it's a 2D shape, warping with a redirect shader on X and/or Z gives most predictable and easy results.
