Hi, A trainee engineer is put in different departments to know the overall process in the company. If the company is small, s/he can be put across all the functions like Finance, HR, Manufacturing etc for a week or two in each. If the company is larger, then the trainee may only see his/her own domain i.e. a Mechanical Engineering Trainee may only be put in Manufacturing, Quality Control, Utilities, Plant Maintenance etc for a week or 2.
So therefore Trainee engineer has to experience almost all departments to finally get the final experience of all so that you are aware about the company work roles and functioning.
Hope this helps.