Minecraft wiki entry for fog, associated text: Must be a string. And it must be either a single word (no spaces) or a quoted string.
Should be in a fog definition name (e.g. minecraft:fog_plains), or fog defined under the Json path "minecraft:fog_settings"."identifier" in the <a_resource_pack>/fogs/<biomes_name>_fog_setting.json file (e.g. fog_plains for plains biome fog).
Specifies the biome fog to display.