You are talking about two different things here. The page you have displayed is showing how the Sonar meters are going to respond in terms of rise and fall times etc. Nothing to do with output level.
Re Output levels. Every interface outputs a +4 dbu signal at a certain db FS level of a test signal. (playing back on a track that is) Interfaces also output a certain voltage level with a 0 dB FS signal as well. It is good to know what it is in fact. Looking at the specs for your audio interface it says that with a 0 dB FS test tone the interface will output very close to +16 dBu. So for an output level of say +4 dBu then you would need a test tone of -12 dB FS. So if you want your interface to deliver a pro level of +4 dBu then you only have 12 dB of headroom to work with. Not bad but not great either.
For 0 dBu output level you would be working at -16 dB FS. Which is about right. This means you have got more headroom now. You could make -18 db FS your ref level too but that means you can only get -2 dBu out of your interface when you are working at that level. Not bad and fine. If you are driving external devices though it might fall a little short level wise.
The nearest K system ref levels for your interface would be K -14. The K -14 ref levels are going to send out +2 dBu from your interface. That is the ref level I would choose for your interface if I owned it. You could also work at K-20 as a ref level as well. You would have a full 20 dB of headroom then. The only thing though is that at K-20 the level that would only be available from your interface would be -4dBu. That is OK also until you are trying to drive some external bit of analog gear which may be expecting +4 dBu. You would be 8 dB shy then and may not drive it well. But at K-14 your interface will still put out +2 dBu which should drive external gear that is expecting +4dBu quite well.
This is important stuff because many have no idea what ref levels (digital that is) produce +4dBu from their interfaces. This info helps you choose a working digital ref level. When measuring the output level from any interface as well you need a true rms digital voltmeter and you need to measure the voltage across the two hot pins of the balanced output as well. eg pins 2 and 3.
Your interface may also offer different output levels too such as +4 dBu or -10 dBv. That will be switchable in the software that comes with your interface or drivers etc.. This is only there so you can match your interface into outside gear more easily. If you are driving active monitors for example from your interface main outs then obviously you set this level to suit your active speakers accordingly.
I hope I have not caused any confusion either. Basically you can choose and work at any digital ref level. It wont effect the levels of your mixes or masters. It just effects the analog output levels that are leaving your interface that is all.