String settings are typically used with variable expansion to configure how something is logged. For example imap_logout_format:

imap_logout_format = in=%i out=%o

The # character and everything after it are comments. Extra spaces and tabs are ignored, so if you need to use these, put the value inside quotes. The quote character inside a quoted string is escaped with \":

key = "# char, \"quote\", and trailing whitespace  "