What technologies use which EL?

What is the difference between a M...

For the EL specification itself, there is no difference between # and $. It is up to the technology using the EL to decide what it means. For both JSP and JSF, expressions that start with a pound sign mean deferred evaluation and a dollar sign means immediate evaluation. This all has to do with when the expression will actually be evaluated during request processing. The pound sign is used in JSF components because we want the expression to be evaluated by the JSF lifecycle and not by the JSP or Facelets engine.