Hi Lugano,
Cat C which is basically ID50 is firm space. Seats are only released when they predict there will be excess capacity. I'm not aware of ZRH being an "Always full" destination (unlike Sydney!!) so I think you should be able to get there in non-peak periods.
The system is all on-line including a Flight loadings and predictions feature which gives you a good idea as to whether trying an ID 90 is worth it.
Hope that helps.
(Am I allowed to say that WTF, or am I exceeding my authority?)