The % character forces OGNL evaluation, so <s:property name="%{foo}"/> will query the stack for a foo property. It's not always required (rarely, in fact), but it's easier to use it consistently for clarity.
The # character accesses a named value stack context variable, for example, you used to need it to access a variable created using the <s:set> tag. You need it to access things like a session variable, like <s:property name="#session.user.name"/>.
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