Hi Pete,
Yes, you would use the Mode C/S (Beta) setting if you wish to detect Mode C and S transmissions, whether as a ground station or in the air, but don't yourself have a transponder. (ADSB out of course still has an active Mode S transponder, so you also need to use the filter).
The '(Beta)' reference by the way is just a reminder that the Mode 'C' detection software is still technically 'under development', but was released (early) because about 40% of all transponders in the UK are still reckoned to be Mode 'C' and consequently lots of our pilot users were asking for Mode 'C' detection. Unfortunately, compared to Mode 'S' transmissions which can be fairly easily identified from the 'Hex' code, Mode 'C' is a far more difficult beast to deal with, though from all reports the system seems to be working well, though we still need to do a bit more work on audio alerts from bearingless targets in general.
Regards
Peter