Upon generation, this block will be 'ignored'. Think of it like setting the alpha of a pixel to 0.
This is useful for defining the bounds of a structure so that it doesn't look like a cuboid when generating.
You can usually go pretty close to the structure with these. Just keep in mind that the blocks 'replacing' this one might range from air, over stone, grass, to even leaves.
Remember you can use Command:space to spare you a lot of work in placing these.
Generates a sort of 'pocket of air' above and below the block before generation of the structure. Useful for creating randomized 'cave' shapes and ensuring there is enough space above the structure.
These stretch to the sides as well as above and below. Usually, you want to place a few of these in the center top part of your structure if necessary. Try experimenting for the best results.