Quick answer
For internationally trained dietitians, Grade B (350) in each of the four OET sub-tests is accepted for HCPC (UK) and AHPRA (Australia) registration. AHPRA's minimum for Writing is Grade C+ (300); Listening, Reading and Speaking stay at Grade B. The Writing sub-test tests your ability to communicate nutritional findings and recommendations to a non-dietitian addressee in 250 words of paragraph prose — not bullet-point plans. Most marks are lost on Genre & Style.