rtexprvalues are complicated a little by the different JSP versions; prior to JSP 2.0, the expression language wasn't understood natively by the container - only the JSTL 1.0 EL library (which provides its own EL parser) used and supported it.
So, in a pre-JSP 2.0 container, rtexprvalues determine whether JSP scripting elements (of the form <% ... %> or one of the JSP document alternative elements) can be used for attribute values.
As of JSP 2.0, rtexprvalues also include EL expressions, which are now natively understood and parsed by the container. Hence, attempting to use either a JSP scripting element or an EL expression in an attribute whose rtexprvalue is false will lead to a translation error.
If you are using a pre-JSP 2.0 container, use the 1.0 version of JSTL (containing two libraries, RT and EL). Otherwise, use the 1.1 version of JSTL (which has only one common library).
Charles Lyons (SCJP 1.4, April 2003; SCJP 5, Dec 2006; SCWCD 1.4b, April 2004)
Author of OCEJWCD Study Companion for Oracle Exam 1Z0-899 (ISBN 0955160340 / AmazonAmazon UK )