The way it was explained to me when I owned a Rolleiflex 2.8 F Planar, was like this.
Lens Name---------------------# of lens elements
Schneider Xenar---------------3 elements,
Zeiss Tessar-------------------4 elements, a symmetrical lens design,
Schneider Xenotar-------------5 elements,
Zeiss Planar--------------------6 elements.
None of these are T*, or HFT coated.
No, it's this way:
Zeiss Triotar --- 3 elements
Zeiss Tessar --- 4 elements
Zeiss Planar 2.8 --- 5 elements
Zeiss Planar 3.5 before ca 1960 --- 5 elements*
Zeiss Planar 3.5 after ca 1960 --- 6 elements*
Schneider Xenar --- 4 elements
Schneider Xenotar 2.8 --- 5 elements
Schneider Xenotar 3.5 before ca 1960 --- 5 elements*
Schneider Xenotar 3.5 after ca 1960 --- 6 elements*
*6 element lenses were used on the Rolleiflex 3.5 E3 and 3.5 F from the third model. The six element Planar and Xenotar aren't said to be better than the earlier five element versions, but the changed were made to make the production of the lenses cheaper. Not sure why, but it may have been that the grinding and polishing of some of the lens surfaces took longer time for the five element constructions.