Hello, it is all quite well explained on the AQA webpage:
Basically, when student dictates, they access third marks for grammar. If they dictate and indicate punctuation, they can access 2/3. For full marks, they would need to dictate, indicate punctuation and spell. I have to admit, that I’m not clear on the “spelling”. It was indicated that they need to spell the “key” or more complicated words. But I never confirmed this with any of the borads?
Hope this helps.