The entire page context which includes the request parameters is available to the included page.
From the standard work on JSP (Hans Bergsten's book published by O'Reilly, 3rd edition, page 560):
All other request unformation is also left untouched, so the target resource has access to all the original parameters and headers passed with the request. Additional parameters can be passed to the target through the <jsp:param> elements in the <jsp:include> element's body.
The only thing you need to be aware of is that the URI you will be operating with inside the included file is that of the including file, which is logical as that's the URI the client will eventually see (it will never know there was any included sources at all, see a single result).