Hi Matthieu,
Bus (and tram) journeys will always be limited by the bus cap in the first instance. The bus fares also count towards whatever rail cap might be triggered, but you’ll only ever pay up to the bus cap on buses/trams. The back office follows the same rules in relation to caps.
Neither photocards, nor blue Oyster cards with a discount attached, go through the back office calculation at the end of the day.