It would have helped if you had posted a link to the displaytag home page at least. Still, it wasn't hard to google. And it wasn't hard to find the documentation for that tag. In which it says that the format attribute gets a string which is a MessageFormat string.
MessageFormat is a Java class, so look up its documentation. Hint: if you want to have different formats for different ranges of numbers then "choice" is the keyword for that. Check out the examples in the documentation.