Unfortunately there is nothing that smart/complicated going on.
For earlier levels – levels N5~N3 I believe – we only allow Cloze-ing from the first 4 sentences, as they have the simpler Vocab in them.
It’s to protect lower level users from being exposed to Vocab that is too hard for them.
We’ve discussed internally a system that allows exposing of those higher sentences, automatically based on user level, or manually based on user settings.
First we need to actually figure out what more accurately what level each sentence is, which requires us to know exactly what Vocab is in each sentence.





I assume the mobile app utilises a different system?