1. "When reply or forwarding messages send to:" refer to "Send to:" address of the original message. i.e. your are recieve email for "help@mydomain.com" for example. Associate "help@mydomain.com" with the persona will use this persona when ever you are reply to a email send to
help@mydomain.com.
2. "Replying to or forwarding messages in folder(s):" use when you file message in specific folders then if you associate that folder, the persona will be used. If you assoicate with "inbox" that kinda replace your default persona