From Documentation
Keep in mind, EL is enclosed by ${}.
${self.label} is EL.
<window>
<button label="ok" id="${self.label}"/>
${ok.label}
</window>
alert(self.label) is not EL, it's a piece of zscript
<window> <button label="ok" onClick='alert(self.label)'/> </window>
Although they both look alike, but EL and zscript have different life cycle, EL is not available at certain phase. Therefore in some cases, you can't use EL but zscript.
<window>
<!-- It's wrong, for java don't accept syntax as ${}-->
<button label="ok" onClick='alert(${self.label})'/>
</window>