While selecting the NOC, the duties and responsibilities mentioned in the NOC should be a close match with the actual duties and responsibilities carried out by you at the respective job. You need not match the duties and responsibilities word by word, but the idea is to have the closest match in terms of the spirit of the duties and responsibilities (at least 60%-70% match).
Also, NOC should be selected on the basis of the match of actual duties and responsibilities carried out by you rather than the designation match in the NOC list. If you are not sure of your NOC, write to noc@workingincanada.gc.ca with your job title, duties and responsibilities and educational qualification, they will help you find the closest fit.
While submitting your application to CIC, you can attach the email that you receive from NOC Team.