My guess, but maybe RME has a different view, explanation to this...
I think the answer is: the default filter of the chip, optimized for highest linearity and lowest latency.
The filter can't be changed for recording interfaces.
To have it configurable is special for ADI-2 DAC/Pro series of devices.
If I remember right, this has to do with the converter capabilities, what's configurable.
For some multi-port converters, the priority is certainly different from to have this configurable.
When looking at the different filter types and characteristics, that are available for ADI-2 DAC/Pro, then I would assume something similar to the SD Sharp filter, from the manual:
DA Filter
Short Delay Sharp, Short Delay Slow, Sharp, Slow, NOS, Short Delay Low Dispersion. The Digital to Analog Converter chip offers several oversampling filters. Default is SD Sharp, offering the widest and most linear frequency response and lowest latency. SD Slow causes a small drop in the higher frequency range, but has a less aggressive (less steep) filter. Sharp and Slow are similar, but have a higher latency. NOS is the filter with the smallest steepness and therefore affecting treble more than the others, but offers the best impulse response. See the Technical Reference section for graphs illustrating the results in frequency response and impulse response.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13
