We expect the space to be filled. But not that every single block is.
Here are some written examples of unfilled plots;
- A plot with a house is the middle and lots of space around it with nothing on it. Or surrounded by loads of bonemeal trees.
- A plot with a little build at the side filling 1/3 of the single plot.
Empty space can be filled with terrain, structure, flowers etc. Just look at some of the examples /warp D, /warp D+.
We also expect a mixture of terrain and structure on plots. Plots can get a grade if one of these is minimal but only a lower grade.