To quote from the Board of Studies website:
"For each question or task (item) in the examination, markers award a raw mark to every student response. In the case of optional items, students’ initial raw marks are sometimes adjusted to compensate for their difficulty relative to the compulsory section of the examination. This process is known as optional question scaling. All of the marks for a particular student are then added together to form a total raw mark for the examination.
Given that student achievement is reported in terms of performance standards, one further adjustment to the total raw marks is made. This adjustment aligns the raw total marks to performance band descriptions.
To achieve this alignment, a group of highly experienced markers, known as judges, decides how a student at the lower borderline of each performance band would respond to each examination item and then determines what raw marks such responses would achieve. These raw marks are then added together for each borderline, producing a recommended set of total cut-off marks.
This process of alignment is multi-staged, involving the review of a collection of materials: performance band descriptions, responses in the Standards Package, statistical feedback, and responses of current-year students consistent with the cut-off marks. Once this process is complete the judges will have determined which total raw marks correspond to the borderlines between each of the performance bands. For example, it may be recommended that a mark of, say, 82 is the minimum required for Band 6, 74 for Band 5, and so on."
To sum that all up, you receive an unaltered raw mark which is then aligned to a final mark based on the standards established by BoS.