Most moderator tools and policies are public, and should be discussed in public for the sake of transparency. But there are a few aspect that cannot be discussed in public, this concerns mostly the tools and policies concerning sock puppets and vote fraud. Revealing the tools at our disposal would make it much easier for users to cheat the system, so this has to stay somewhat secret by necessity.
The policies surrounding sock puppets and vote fraud are not actually secret, but it is rather hard to formulate them in a way that doesn't reveal specific information about the moderator tools. So while the general policies are public, there is still a lot of information that would be very useful for new moderators that is not available at any easily accessible place.
The way it works at the moment is that a new moderator just asks in the moderator-only chat room when they have such an issue and some moderators or SE employees will walk them through the process. One significant disadvantage here is that regular moderators don't have access to the tools on other sites, so we have to advise blind, as we can't really see the information the moderators on-site see. It is far easier for comm team members to walk a mod throught the whole procedure, but they are not always available and very busy in general.
I propose to have a part of the new help center that is only accessible to moderators. The content of that help center would be identical across all sites, and it would be created by the community team and the moderators. I'd let moderators suggest edits and let the comm team have the final say on the page, but other variants are possible. This mod-only help center would host pages on topics that can't be discussed in public without compromising the effectiveness of our tools. I can only think of two topics right now, sock puppets and vote fraud, but there might be more. But even those two topics alone are rather important, and having some clear guidelines would be very useful.