Sonnar is a trademark owned by Zeiss AG, they use it how they want for any lens of some speed. A Sonnar type, as invented by Ludwig Bertele in the 1930's, ist a triplet-derivated (asymmetric) design consisting of 6 or 7 elements, 3 optical groups and having at least one cemented triplet.
In the 1950's some manufacturers like Canon and Pentax experimented to dissolve the rear-group of the Sonnar type, creating something in between Sonnar and Planar. In the 1960's Zeiss produced a set of f/2.8 telephotos with 4 or 5 elements, also calling them Sonnars.
The 2006 C-Sonnar is more a Sonnar than any other 50mm lens produced in the last 45 years, even if it has just 6 elements (3rd element cancelled) in 4 groups. The rest still looks like the 1932's Bertele design. It still has the cemented rear triplet
http://www.taunusreiter.de/Cameras/Canon_RF_e.html#Update
A detailled comparison will follow.
cheers Frank