The special meaning of the parameter. You shouldn't use one that has a meaning you could override by mistake somehow. The semicolon is part of the standard. But doing something like <code>/css/my-css-i-am-a-hash.css</code> will work too. But I like hashed at the end for an easy globing on the cache config (<code>*;AM*</code>). But I've changed it to ``;AM-i-am-a-hash`` to prefix it with the AssetManager naming. Akamai do the same with ``;AK-i-am-a-hash`` and F5 by ``;WA-i-am-a-hash`` - it's a detail that can be changed however you like :-)