required quite simply implies what it says. The attribute is required or mandatory.
rtexprvalue means Runtime Expression Value. It means the attribute can support scriptlet values.
elexprvalue means it can support EL (expression language) values.
So, if you have requiredattr defined as both required=true and rtexprvalue=true and elattribute is defined as elexprvalue=true, you can write as follows:
<myprefix:mytag requiredattr="<%=baz.getId()%>" elattribute="${foo.bar}"/>