It's core Python, but it's specifically intended for producing redistributable installers containing Python packages. This is not something that the embedded distro is meant for, and it neatens things up to omit them.

There is very little information because it's considered an internal implementation detail (otherwise it would be thoroughly documented). That said, because of the narrower use case for the embedded distro, omitting even thoroughly documented modules would be okay if it didn't make sense in this context (e.g. the turtledemo module is documented, but also not in the embedded distro).