From the API Spec for getParameter:
Returns the value of a request parameter as a String, or null if the parameter does not exist. Request parameters are extra information sent with the request. For HTTP servlets, parameters are contained in the query string or posted form data.
As far as I know, the form data is automatically put into the parameter list. From the description above, it sounds like there might, potentially, be some other information there, as well.
You can, however, add whatever objects you'd like to the request through the use of the setAttribute method. There is no setParameter method so, if you're ever trying to retrieve something from the request that you put there,
you should be getting it from the attribute list.
I hope that helps,
Corey