What are some things that a C1 can do that a B1 cannot?

Too hard to classify it objectively. I would say that C1 speakers are more aware of what they don't understand and, therefore, they can search for answers without external help. Also, B1 speakers sometimes are too concerned about grammar rules. Grammar is important of course, but it should be used to "polish" your English once you have learned how to communicate.

Avoiding comma splices etc would be one differentiator.

Break rules correctly

Although I didn't take a formal exam, I consider myself to be in the B levels. So, speaking for myself, some things I think I can't do is debate or argue at the same level as in my native language, either verbally or in writing, like when discussing politics or writing essays. I consider these to be advanced or maybe I'm just lacking practice and formal study.

Quantum Field Theory

One big difference is the listening ability. A person might be good at grammar or memorizing words. But the true measure of a skill is how well one can comprehend native speakers These are two videos that that can test a person's skill! One is a beginner video, and one is an intermediate video. [https://youtu.be/tFFe6vnvlPQ?si=Uw6qJq3dWvMvLeKU](https://youtu.be/tFFe6vnvlPQ?si=Uw6qJq3dWvMvLeKU) [https://youtu.be/a2RciGNkrF0?si=xZzD7TYaZXOfm8xS](https://youtu.be/a2RciGNkrF0?si=xZzD7TYaZXOfm8xS)

Change accent or regional dialects

I'm a C1, at least based on test scores and such. I think in English when I use the language, so yes, that might be one of the things C1 can do that B1 can't. If you want a comprehensive list, you can start by looking at something like the IELTS scoring criteria. It shows how you are graded in different aspects of the language. Example: IELTS Speaking Band Descriptors: https://takeielts.britishcouncil.org/sites/default/files/ielts_speaking_band_descriptors.pdf IELTS Writing Band Descriptors https://takeielts.britishcouncil.org/sites/default/files/ielts_writing_band_descriptors.pdf

[There’s a whole official CEFR PDF describing what a person can do at each level.](https://rm.coe.int/common-european-framework-of-reference-for-languages-learning-teaching/16809ea0d4)

They can handle a conversation with multiple participants where people change the subject frequently.

Being able to start a sentence without knowing how you're gonna end it

Better question would be what does a C1 do that a B2 can’t, but there is still such a huge difference. B2 is when you’re considered competent enough in the language to manage everyday life, once you get in the Cs you are fluent.

They'd probably say "can't" instead of "cannot" in a reddit question. And "I've heard" instead of "I heard some things". And not repeat "some things" twice in the same sentence.