There is no confusion, you simply may not be familiar with different line levels. There are "consumer level" line levels for typical consumer devices, as well as different levels for studio equipment, as there is not one common standard.
Here are some general aspects to consider:
Reference levels on outputs refer to the output volume, with higher levels being louder.
Reference levels on inputs refer to the sensitivity of the input. Higher levels make the input more insensitive to support hotter signal levels and peaks.
It is better to have a little reserve on inputs to prevent overloading. In the digital world, nothing should go higher than 0 dBFS.
Check the manual, as some interfaces may have only 2 reference levels on input, while others have settings for trim gain to add a few dB of more gain/sensitivity. This helps match the input signal, especially for lower signal levels like consumer level.
Regarding UCX II
Input sensitivity is switchable to +19 dBu and +13 dBu
Variable gain ranges from 0 to +12 dB
To achieve sufficient sensitivity for consumer level, select +13 and add 12 dB of gain. It's good to have enough headroom, but not too much; keep an eye on the input meters' level. For live recording, keep a higher reserve compared to recording known material where the max peak is known
Switchable between +19 dBu, +13 dBu, and +4 dBu.
For connecting active monitors, select +4 dBu to avoid excessive volume.
For connecting external devices, start with +4 dBu and check if it results in a satisfactory input level on the device. If it has a volume knob, turn it to about 75-80% and check whether +4 dBu is sufficient. If more volume is needed, select +13 or +19.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13