What do these D's mean?
So in my dump some items are marked with a pink D. Some are not. What does it mean?
Solution 1:
This means they have been marked for dumping, and your dwarves with the garbage hauling labor enabled will haul it off to your garbage dump as soon as possible.
You can select them like you have in your screenshot and press d to toggle dumping, or use o-d-b to do it to an entire area.