The setting influences buffer/latency. The only thing I can't determine is if the card really plays back on 24 bit or 16 bit.
thats unlikely that a card has a 16 bit AD/DA mode in addition to 24 bit – but i dont know RME enough. it might be that 16 bit is available for ADAT out, for example.
Normally you would choose the playback resolution in the AISO driver.
normally you wouldnt have different resolutions in one converter.
the converters from motu, m-audio, sony, eventide, mykerinos, usbaudio, terratec, avid, apogee, eventide, tc, and digidesign always only support 16 OR 24 bits format.
though i remember that at least one of the nuendo branded 19" units also had different modes (and they are hammerfall based.)
some of the high end sony cards and the weiss converter also have different modes - but of course at different ins and outs.
So does it play 24 bit automatically if I e.g. set record to 24bit in Cubase? Or?
nah, the setting of the software is independant from that. the setting of the host program only affects reading and writing files, everything releated to the card must be available in the ASIO controls.