In logic subjects, it is common for there to be answers that are uncontentiously right or wrong. Getting the right answer, or at least a close approximation to it, is then a necessary condition for being given marks. For the exact meanings of these grades, you should consult the University Regulations. At the end of a subject, all your work is reviewed at an examiners' meeting, and a final mark is determined. Normally, this will be obtained by an averaging process over all the marks of the semester, but in borderline cases decisions will also take into account other factors, such as tutorial performance.
Examiners do have the power to raise students' marks, even in non-borderline cases.
This is done in exceptional cases, such as where illness has clearly adversely affected a student's mark, or there are other reasons as to why a simple averaging would not produce a fair mark. In some subjects, especially the logic ones, individual pieces of work may be given a percentage. This is transformed into a standard grade at the end of the semester. The conversion procedure may vary from subject to subject, and may be discussed with individual subject co-ordinators. If you think this may be necessary, contact the School Office or an appropriate staff member.