The CEFR organises language-use reference levels as A1, A2, B1, B2, C1 and C2. The A, B and C groups describe basic, independent and proficient use respectively. Descriptions concern what someone can do under particular conditions; they are not a universal count of known words.
For study navigation, think of increasing independence: handling short predictable exchanges, sustaining conversations and texts across more topics, and interpreting or expressing finer distinctions. This is a broad orientation, not a table for assigning yourself a level. Use the official descriptors and the relevant skill when comparing courses or assessments.
For example, reading “Your booking is confirmed” and identifying a reservation does not establish A2 or B1. The text is short, the topic may be familiar and the response may have been memorised. Explaining why a booking needs to change requires another sample. C2 does not mean knowing every profession, slang expression or culture. When a course label tells you little, examine the tasks and examples it actually teaches.