The Primary ID Parking feature is for your main Yahoo ID that you use.  You can use this if you are going to be away and it will record any IM's sent to you as well as DETECT if you have been scanned by YInformer!  The image below will show you what it will return if you get an IM or scanned.

Click on an event in the larger box to have a popup message with the details.

We are working on having a customizable auto reply for IM and scans but could take a while to implement.

Extra:

The Primary ID information will be remembered, so you do not always have to re-enter it in over and over.