A number of solid blocks with no specific usage apart from being used in structures and structure transformer scripts. Blocks that have default transformer functionality are named and listed here.
Natural Floor Edit
Upon generation, this will ensure that your structure has a decent basis to stand on. It will generate a sort of diamond shape below it if required, trying to imitate natural floor design.
Can also automatically be placed by Command:floor.
Usually, you want to place this directly below your structure, with a little bit of fuzzy offset to it. The type of the block is selected from the surrounding blocks and the biome it is in, so it's fairly reliable. Keep in mind this can also be stone, so if you need grass, place grass on top, and use the natural floor below.
This block will disappear upon structure generation, being replaced with air. This is useful for keeping entities in place until the structure generates, by surrounding them in a wall of barriers.