JSP ${param.id} usage _jsp programming

Its range of values is page,request,session,application.
${param.id} is related to input, relative to Request.getparameter ("id"). Gets the parameter ID of the input.
Because the a.jsp is committed to the b.jsp process, the ID does not belong to one of the page,request,session,application ranges, so it cannot be taken.
And the ID is a parameter that a.jsp passes to b.jsp, so ${param.id} takes the value.
The scope of the request refers to a JSP page that sends a request to another JSP page.
In a.jsp <%request.setattribute ("Key", "Test")%>
When a.jsp turn to b.jsp, b.jsp can get the value of the a.jsp variable key, while the other JSP pages are not.
The key here is a.jsp to turn to b.jsp, in a.jsp plus <jsp:forward page= "b.jsp"/> said to turn to b.jsp. Otherwise b.jsp cannot get the value of the key that a.jsp set.