This is the version you will likely get if you have allowed your game to update, but have not opted into the latest, more recent patches (like 1.6.1130 or 1.6.1170). It's considered the "current Anniversary Edition build" for tools like the Skyrim Script Extender (SKSE), making it a safe and reliable choice.
The "Skyrim Special Edition 1.6.640 Verified" movement represents the modding community taking control of its own software ecosystem. By freezing your game state at 1.6.640, you gain access to a golden age of modding stability, combining the modern content of the Anniversary Edition with the bulletproof infrastructure of a finalized, highly compatible game version. skyrim special edition 16640 verified