This is a library to share functions in Go across packages. Function that are generic enough should fit right in here. If you're not sure what that is take a look around, hopefully it will become apparent.
What is meant by sub-package here is putting related functionality into subdirectories.
We do not want to fracture the code into too many packages, nor make one big
monolith. Try and group related functionality. If your not sure, then just
place them in the stdlib package. These things tend to work out over time.
Functionality can always be moved later. Yes, moving them into another package
later on will force a semantic major version increase. But don't worry
about that, just get it done, and we'll figure it out.
See What to put into a package for further assistance.