The params scope contains all of your request's parameters, including all url and form variables, and any computed values created by the current request's route.
The flash scope is a temporary persistent scope.
Any data that is put into the flash scope is automatically persisted until the next request, where it is available inside the requestContext or using the flashKeyExists and getFlash helpers.
The flash scope is very useful for displaying validation messages to the end user. For example, to notify the user that a record was updated successfully.