For miscellaneous discussions and remarks not suitable for top-level posts even in the Discussion section, let alone in Main.
(Naturally, if a discussion gets too unwieldy, celebrate by turning it into a top-level post, just like in the good old days.)
Is there a way to rss subscribe to my message inbox, or to get emailed when someone replies to my posts?
There is an rss feed for the message inbox. Although, I can only subscribe to it using Firefox's builtin mechanism, not the external RSS reader I use.
ETA: I presume it relates to the authentication cookies LW uses to identify whose inbox to show, which Firefox has access to (since I'm logged on), but the external reader doesn't.