Writing to err vs std

I see things like git fetch and other processing writing details to stderr and not std. Can someone point to why this is and what the rule is for determining when to write to err vs std? See 165f390250874b32ed8158150fe49d574297c9f9 for example. Is there a standard in the git project that says write this to err while write that to std? Any guidance and why would be appreciated.

Thanks, Jay Asbury