To be clear, the examples are from other posts online, not official examples.
I think its a mistake to assume that because documentation is complete that it is adequate.
It would assist users to discuss this common task in the section on the optimiser rather then them have to find it in a long list of other functions and have to realise its relevance. I would expect most users want to restrict the frequency range of their goals. The way QUCS Studio does this is different to ADS and other similar packages, so a helping hand may be in order.
Another issue with the optimiser in QUCS Studio is that it conducts its business in private. ADS and MWO both show the intermediate steps which can help to understand how to hone the goals to get what the designer wants. The optimiser can also be interrupted and variables tuned or their range restricted if the optimiser wanders off into the weeds.