XPPF Filters are the Plug-in filters, I believe. Look at these links, but PLEASE make sure it's what you're looking for before you order.
SmartHome.com -
http://www.smarthome.com/4845.HTMLsmarthomeinc.com -
http://www.smarthomeinc.com/X-10/x10_error_prevention.aspHomeTech -
http://www.hometech.com/plc/couplers.html#6288Roland