According to the HEO (ch 5 p 2), if you are employed on a doctoral studentship you may use up to 20% of your time on departmental duties. These duties should be included in the Individual Study Plan, the General Study Plan, or both. The time spent on such duties does not count towards your study time as a doctoral student, which means that you get prolongation for the time spent on such duties. Normally, this time can be spread over the four years of your education (two years if you are pursuing a licentiate). If you have departmental duties for 20% of your time, your employment will correspond to five years of full time for a doctoral degree.
The most common departmental duties are teaching and supervision, but they may include other educational tasks, administrative tasks, or research tasks. Teaching is usually done as a teaching assistant, where tasks include leading exercise or lab sessions and grading exams and lab reports, but it can also be done as a lecturer or course coordinator. Supervision tasks can involve co-supervising bachelor or master projects.
The type and amount of departmental duties assigned to a doctoral student on a doctoral studentship is formally decided by the university. However, they are often decided in agreement with the student.
If you are not employed on a doctoral studentship you may also have similar responsibilities. Your financing will also be extended for such duties.
If you are employed outside of the academic sector you may have such responsibilities at your employer or at your university. Since the HEO restricts such duties to 20% only for doctoral studentships, on a different type of employment you may have a larger percentage (always with a corresponding prolongation of financing). However, since you are still expected to finish with a doctoral degree within eight years, these duties cannot amount to more than 50% overall.