Hi, that would depend on the university and department.

In mine, you can just be randomly allocated by the technical staff, or your supervisor/another academic can request you specifically/you can request what you want to do. It would be usual that the 3rd years get more difficult practicals in some cases ie if the academics were allocating the practicals they might want more experienced students. If the technicians are allocating, they might not understand who is what year, or the differences in difficulty of the practical, so it may be more random.

Oh ok, that makes sense :)