Sets which language you're answering in — flash cards, multiple choice, and text entry all quiz this way. In Japanese mode, questions mix English→Japanese translation with kanji↔kana reading practice (for words that have a kanji form).

Multiple choice only. Every question is about one kanji (or a run of kanji) from a word — 食 from 食べる — and the kanji is either the question or the answer: kanji → reading, kanji → English meaning, reading → kanji, English → kanji. All four options are always the same kind (all readings, all meanings, or all kanji). Words with no kanji are left out. Applies on top of your Word set and Category filters.

Words you've missed, answered slowly, or barely seen are more likely to be picked — and tend to come earlier — than words you're solid on. A soft bias, not a filter: everything in your filters can still appear.