Band 8 (Very good user) pushes to the top of C1, fully operational command, nuanced expression, and rare errors only in unfamiliar situations.
High-advanced. Band 8 corresponds to the upper half of CEFR C1.
Band 8 is a very good user with fully operational command of the language, marred only by occasional unsystematic inaccuracies and inappropriacies, who handles complex and detailed argumentation well. The vocabulary is wide and precise: idiomatic items used naturally, sophisticated collocation, and confident control of style and register. Mistakes are rare and unpatterned rather than systematic. This sits in the upper half of CEFR C1, the edge of advanced proficiency.
You're at band 8 if you express subtle and abstract ideas with the right word almost every time and rarely misunderstand even dense or implicit text. Band 8 is asked for by the most selective universities and a few professional registrations, and it is well beyond ordinary admission requirements. At this level the remaining errors are individual lapses, not gaps in knowledge. Pushing toward band 9 is less about study and more about eliminating the last unsystematic slips.
At band 8 broad coverage is largely done, so target the specific idioms, fixed expressions, and register choices you still get slightly wrong, and keep them in spaced review until they stop recurring. Read and listen to demanding native material, literary essays, specialist journalism, to absorb the idiomatic precision that marks a near-native lexicon. Have advanced writing critiqued for the rare inappropriacy rather than for range, which you already have. Notice and log every word you use that is almost right, because closing that gap is the whole task now.