There are a lot of internal university processes that relate to quality assurance to make sure marks are given fairly, transparently and without bias.
For example, all assessment and marking are overseen by a Board of Examiners (BoE). They also approve final marks and are consulted in the results moderation and verification processes as well as the re-scaling and/or standardisation of marks.
Another example is that the person who enters the results for the assignment is not the same person who verifies (or double checks) if the results were entered correctly.
And another example involves failed grades, which is considered separately from the rest of the grades for a particular assessment. Double marking is required for failed assignment tasks.
Finally, where practicable, there is scope in the policy for anonymous marking. Of course, if it is a placement or an oral presentation, then anonymous marking isn’t practical and where possible the BoE should plan for two markers to be present during the performance or oral presentation. Placements, of course, are a little more difficult.
As you can see, as a student learning from the tutor or subject coordinator, you will need to mount a convincing case for why you think your mark was not a fair one as they are tasked with having subject matter expertise.