A structure generator will spawn another structure on generation. For example, if you have a tower where each story is randomised, you include a structure generator script to randomly spawn in one of your pre-built stories to spawn.
If you want to use a structure (or randomly select one) in your builds, place one of these down, and right click to edit. It will generate the structure from the lower corner out into +X, +Y, and +Z.
Advanced Mode Edit
This mode will spawn random structures from a weighted list. In other words, make sure the list ID from this generator matches up with list IDs from Structure List spawn modes from other structures.
This will randomly choose a structure to spawn from that list, making it extendable with any amount of structures.
Simple Mode Edit
This mode simply spawns a structure by ID.
If you want it to choose between multiple structures, separate them with commas.
Example: "TowerGreen,TowerGreen,TowerRed" will give TowerGreen 1 2/3 chance of spawning, and TowerRed a 1/3 chance.
Make sure there is enough space for the structure to spawn, too.
Note that this block is designed to spawn a structure on spawn of another structure, so that you can generate randomized structures as part of others. To spawn a structure normally, see Guide:Building a Structure.