"Rule Airspace" is the airspace defined in
FAR 91.225, in which ADS-B out will be required after 1/1/2020. Class E would qualify if it is above 10,000' MSL (and more than 2,500' AGL), or above the ceiling of Class B or C airspace, or within a Mode C veil around a Class B airport. That's why I had to stooge around for half an hour within the confines of the lateral limit of the Portland Class C, but in E airspace above it.
I didn't understand that at first. The first time I flew after the equipment was installed, I just flew around in Class E airspace, away from the Class C, and below 10,000'. I submitted the request for report from FAA, and it said I failed because there was not enough time flown in the required airspace. So I had to go up and do it again.