This process is used to lift Part Lot Control restrictions from lands within registered plans of subdivision to create parcels for sale. It usually is used for townhouse developments after construction has started to accurately set the boundary lines between the townhouse units. Section 50 of the Planning Act grants the municipality the authority to pass a By-law to exempt lands within a Plan of Subdivision from the Part Lot Control provisions in the Act.
Source
http://www.toronto.ca/developing-toronto/pdf/guide_sectionE.pdf#section_e