Band 9 (Expert user) is the C2 level, near-native precision, idiomatic fluency, and complete command of the language.
Near-native / proficient. Band 9 corresponds to CEFR C2. Only ~1% of IELTS candidates score Band 9.
Band 9 is an expert user with full operational command of the language: appropriate, accurate, and fluent, with complete understanding. The vocabulary is effectively native-like, idioms, nuance, register, and the least common items all handled without strain or error. There is no longer a meaningful gap between what the candidate wants to say and the word available. This is CEFR C2, the ceiling of the scale.
You're at band 9 if your English is indistinguishable from an educated native speaker's across every skill, with errors essentially absent. Only around one percent of candidates score an overall band 9, and even strong C2 users often land at 8 or 8.5 because the band is unforgiving. Almost no institution requires it, so it is pursued for mastery or professional prestige rather than necessity. There is no higher band to target.
Preparation at this level is about exam technique and consistency rather than learning new words, since a true C2 lexicon is already in place. Use spaced repetition mainly to retain rarely used precise items and specialist terminology that even advanced users let lapse. Read and write across genres you rarely touch, legal, scientific, literary, to keep the full range live. Do timed full mock tests under real conditions, because at band 9 the difference between 8.5 and 9 is usually performance management, not knowledge.