Here it is:
>>> Regulated Occupations -
If you are working in a regulated occupation that requires mandatory certification or licensing in Ontario, you must submit proof of your Ontario certification or licence. You may wish to contact the Regulatory Body for more information about becoming licensed in Ontario.
SOURCE: Regulated Professions - http://www.ontarioimmigration.ca/en/working/OI_HOW_WORK_PROF_PROFS.html
If you are working in a regulated skilled trade that requires a Certificate of Qualification in Ontario, you must submit proof of your Ontario Certificate of Qualification issued by the Ontario College of Trades.
SOURCE: Certified Trades - http://www.collegeoftrades.ca/
PLEASE NOTE: The above statements only apply if you are currently working in Canada in an occupation that is regulated in Ontario or you have arranged employment in a regulated occupation in Ontario.
If you are applying from outside of Canada, and you do not have arranged employment in Ontario, you are not required to send proof of certification, licence or a certificate of qualification issued by the Ontario College of Trades.