Section 1 is marked by Cambridge University and the standardisation of the scores is undertaken by them, before they send all the results to the University of Oxford. Section 2 of the TSA is reviewed by the admissions tutors of the Oxford College you have applied to. It is thought that the essay is marked in a similar way to undergraduate essays at Oxford, where 60 or more represents a good performance, and 70 or more would be very good. However, it is not entirely clear how the standardisation of the essay score takes place.
Important to note is that although the essay is marked out of 100, it doesn’t mean that it is weighted double that of section 1; in fact, section 1 is usually weighted more highly.