The overall band score is broken down to 4 criteria:
1 - Task Achievement (do you answer a questions / task in full, address all parts of the task, present clear arguments that are connected to the initial topic)
2 - Lexical Resource (do you use a wide range of vocabulary with very natural and sophisticated control of lexical features)
3 - Coherence & Cohesion (do you manage "paragraphing" skillfully and use cohesion in a natural way, is there a clear progression of ideas, are sentences linked to each other, is it easy to follow the storyline)
4 - Grammar Range (do you use a wide range of grammartical structures with full flexibility and accuracy, are your sentences error-free, do you showcase skilful punctuation and complexity in sentences).