What specification does the %{...} expression in JSP refer to?

Question

Java

I study J2EE using the example of a finished project, got acquainted with Servlets, JSP, Custom tags, Struts2.
But in JSP files I see constructions like this: I understand that this is getting the value of a servlet property, but I can't find any documentation on this construction. Tell me, what specification does the expression % {...} refer to?
<s:if test="%{testProperty}">

Answer 1

Actually, no %{...}, but ${...}.
In my opinion they appeared in version JSP 2.0