Minecraft wiki entry for Development_versions, associated text: Added two new variables to the biomes_client.json file: remove_all_prior_fog and inherit_from_prior_fog, which control fog merging and/or inheritance.
remove_all_prior_fog, when true, will clear all previous fog definitions stored on the stack, making the current pack the new "starting point" for fogs.
inherit_from_prior_fog, when true, will merge a biome's fog definition with matching biomes on the stack then create a new merged definition for it.
transparentattachable tag should only affect rendering in first person camera perspective.