Re: JSTL example

I'm reading now the JEE5 Tutorial and in a JSTL section, there is a
code:

<c:if test="${!empty param.Add}">
  <c:set var="bid" value="${param.Add}"/>
  <jsp:useBean id="bid" type="java.lang.String" />
   <sql:query var="books"
    dataSource="${applicationScope.bookDS}">
    select * from PUBLIC.books where id = ?
    <sql:param value="${bid}" />
  </sql:query>
  <c:forEach var="bookRow" begin="0" items="${books.rows}">
    <jsp:useBean id="bookRow" type="java.util.Map" />
    <jsp:useBean id="addedBook"
      class="database.Book" scope="page" />
  ...
  <% cart.add(bid, addedBook); %>
...
</c:if>

 my question is what is the purpose of

<jsp:useBean id="bid" type="java.lang.String" />

, in

<sql:param value="${bid}" />
what I am referencing: the bean "bid" or the variable "bid"? It is a
little bit confusing for me. Any helps?