Sorry if everyone else knows about this stuff, but I can't find any information about it anywhere.
I see occasional comments about one user putting something on another user's "talk page". I have some questions for a particular user that don't rise to the level of an official question (but it's not a private matter), so I'd like to use this feature. But I can't find it anywhere. What is it? Does it just mean that someone has opened a chat and invited or tagged the other user or something like that? (If so, are there best practices for that?) Or are they friends in some other venue?
Edit:
My reason for asking this is that I really like one person's formalism and want to ask that particular person for references the she/he uses.
To be clear, I understand that chat exists, but I particularly want this one person's input, and it looks like chat doesn't really have a mechanism for that. I certainly don't mind if other people chime in with their opinions about references, but I would like to ask one person in particular.