I think the terminology used by Zeiss is based on optical design!

SONNAR lenses are triple group design, the front group has a single element, the second has two, divided by one "fill in" between them, and the rear one has a double or triple component.
Other companies using the same design refer to it as "Sonnar type".

However, it`s seems they can call HOLOGON to any deep-set super-wide optic, BIOGON to any deep-set wide-angle, DISTAGON for those that aren`t deep-set, PLANAR to high-speed normal or semi-wide lenses of symmetrical optical design,TESSAR to medium-speed ones and SONNAR to longer focal lenght lenses.

http://www.zeiss.de/C12567A8003B58B9...256A53003735CE


Cheers

André