Exports the current selection to a structure (including entities, blocks and tile entities). You can also specify a name to copy another structure's spawn data. This is especially useful for making small changes to a structure, and then using the previous version's spawn data to just overwrite its world data instead.
Opens a GUI to edit the metadata when executed.
Remember, you can also use this do save your progress on a structure, since you can later 'just' edit the relevant parts by using either Command:import to edit the world data, or Command:editstructure to edit the metadata.