Q: changing category of multiple clips

Quoting John F. Whitehead
"Create a Schedule with a Search Expired response to trigger a Custom event that you create a Subscription to that triggers the archive response. Got it?"
See threads
http://discussions.apple.com/thread.jspa?messageID=11629122&#11629122
and
http://discussions.apple.com/thread.jspa?threadID=2260768

I am using it all the time. Took a while before I got my head around it, but it works like a dream as long as there is a way to filter the assets you want to work on.

As previously noted, you can use the Search Expired response to batch other responses. Create a new Search Expired response and set it to use the Custom 1 event type. Set the Timestamp Field to "Creation Date". Leave the Expiry Days empty. Below that you can filter based on the usual options or leave them all open to hit all your assets (up to you).

Next, create a Set Asset Metadata response and populate it with the value you want to apply to your assets. Set this response to run by Subscription based on the Custom 1 event type.

Finally, schedule your Search Expired response to run. When it does, it will fire what amounts to a dummy event (Custom 1) on the filter it was set for, triggering the Subscription you set up to listen for that event type. That fires the Set Asset Metadata response, and now all your assets are tagged with the desired default in the new field.

The Custom event types are there for tethering a Search Expired response to a Subscription, so unless you know what you are doing, avoid the other event types on a Search Expired response.

The official documentation does not really explain this very well and it is a bit wonky, but it does work. Once you get the desired result, you can disable the schedule for the Search Expired response.