Minecraft wiki entry for Development_versions, associated text: Block variant {"nbt": path, "block": "coordinates"}, where coordinates field uses same format as /setblock.
Entity variant {"nbt": path, "entity": selector}, where selector field uses same format as /kill.
Additionaly, if field interpret is present and set to true, contents of selected tags will be interpreted as chat components.
Works same as selector components:
If there are no elements, returns empty string.
If there are multiple elements, merges them with ,.