It is part of the Erlang's standard library. SSH and SSL state and connection management has been re-written recently to use this built-in module.

Here is a video of Raimo Niskanen, the author, describing the module:

https://www.youtube.com/watch?v=f_jl6MR3kXQ

The title is "The tool that you never knew that you always wanted", which was borrowed from another member of the community (Jesper L. Andersen)