OK, I just had to look this up, because I didn't understand it myself... hope it makes sense!
As I understand it, in the third year, you take a paper called 'Special texts', of which there are 5 options, all involving translation from Modern/Classical Chinese. You also take a Special Language paper, which is in Japanese, Korean or Linguistics. Or instead of either of these papers, you can take an additional language (Japanese, Korean, Tibetan) which comprises the full subsidiary.
So, it works out as:
No subsidiary language = Special texts + Japanese/Korean/Linguistics
Subsidiary language = No Special texts or Special Language paper, just additional language
I know that technically the 'no subsidiary' option DOES involve a subsidiary language, but because the language level you reach is low compared to the 'subsidiary' option, it doesn't count.
If any of that didn't make sense, ask and I'll try and explain it clearer!