.SSA, .SSE, and .SSS files all use the exact same format, and as far as is currently known all do the same things. The only differences appear to be which links exist. These files and their respectively named .SSB files are effectively linked, as within the script they are not called by extension, but by name.
These are never found alone. They are like C/C++ .h files, in that they are connected to a certain file, and therefore useless by themselves. They are used to determine the starting positions of Pokémon, backgrounds, and interactable areas (including ones that you simply move into t