The National Occupational Classification list is a database that is developed by the government of Canada. It categorizes all occupations in the country present in the Canadian labor market.
Express Entry: If you want to immigrate to Canada through Express Entry you must be grouped in must be skill type 0, A or B.
Atlantic Immigration Pilot: If you want to immigrate to Canada through the Atlantic Immigration Pilot your work experience must be skill type/level 0, A, B, or C.