The development file for everything currently being worked on that is not in a dedicated section file (in active development) or in TR_Mainland (released). There should be few, if any, edits in this file.
Do not edit the edges of the file without loading the section files adjacent to it, and cut the cells from them that you edit back into the adjacent sections when finished. For the western edge, you need to load Skyrim_RestExteriors.esp or the Orcish Switzerland Heightmap instead of a normal section file.
There are still miscellaneous non-exterior things left in this file: StuffInRestExteriorsThatAreNotExteriors.xlsx
search and replace operations to change the old cliff meshes with our newer ones, and long term the cliffs made from parts of the Mortrag Glacier, retextures of the glacier, and useages of the glacier itself need to be replaced.