I'm using the Message module on Drupal 7. For Replacement tokens, the help text says:
A comma-separated list of replacement tokens, e.g. %title or !url, of which the message text makes use of. Each replacement token has to start with one of the special characters "@", "%" or "!". This character controls the sanitization method used, analogously to the t() function.
What's the difference between %, !, and @? Are there standards about when I should use each symbol?