I saw your comment a while ago but I didn't say anything because at the time I did not know the answer!

But, in my research for tomorrow's full PlanBuild tutorial video + article, I discovered the information you need so I will share it :)

The terrain modification marker is one of the marker items, such as center point or snap point markers, which you place on your building sort of like you are adding another build piece. When you save the blueprint with the marker, the marker will vanish and be applied to your blueprint. So, when you place the blueprint, the ground will be adjusted differently depending on how high up or down (z value) you place the blueprint. If you place the blueprint below the ground, the terrain markers will cause the ground to make a hole. If you place the blueprint above the ground, the terrain markers will cause the ground to raise to make a cliff.

For the terrain modification marker, it will bring the ground up to in that specific spot. You can further specify the radius of the terrain, as well as the paint it applies. Here are the variables. If I remember right you do this by placing the terrain marker, then selecting it to change the variables. (Taken from Planbuild mod page)

• Shape: Set a circle or square shape.
• Radius: Radius of the terrain mod operation.
• Rotation: Rotation of the marker (only makes sense on the square marker).
• Smooth: Add smoothing to the edges of the terrain modification.
• Paint: Optionally set a new "paint" (Grass, Dirt, etc) to the ground when modifying the terrain.
  

For your dock, you would place the terrain modifier where you want the raised earth support to be. You can kind of imagine the marker like an instant hoe, its like you leveled the ground, lowered it, or raised it to that specific spot.