You are adding an additional category rating (defined in part 1.1) to a commercial certificate. The reg trail starts with "61.63(b) Additional aircraft ratings (other than on an airline transport pilot certificate); Additional category rating. An applicant who holds a pilot certificate and applies to add a category rating to that pilot certificate: (1) Must have received the required training and possess the aeronautical experience prescribed by this part (in the case 61.129) to the pilot certificate for the aircraft category (in this case 'airplane') and..."

This leads to 61.129 which spells out the flight training and experience requirements that must be in airplanes - 61.129(a) - as stated above. As Rick said the actual tasks to be accomplished on the checkride are spelled out on page 1-v of the Commercial Pilot PTS under the column heading of "RH".