C1 corresponds to the advanced level. According to the CEFR, students at this level can...

• Understand a wide range of demanding, longer clauses, and recognize implicit meaning.
• Express ideas fluently and spontaneously without much obvious searching for expressions.
• Use language flexibly and effectively for social, academic and professional purposes.
• Produce clear, well-structured, detailed texts on complex subjects, showing controlled use of organizational patterns, connectors and cohesive devices.