Import from feed using a selector only if it has a given XML attribute
I figured this out with the help of an XPATH cheatsheet that shows how selectors I might have written to target HTML would transfer to XPATH values.

To only pull &lt;category domain="keywords"&gt; items, I can use category[@domain="keywords"] as my selector. The other set of &lt;category&gt; items don't have a domain attribute, so I pulled them separately using the selector category[not(@domain)].