"It is important to start the commissioning process in the pre-design phase. This early involvement is critical for the timely and useful development of the Owner's Project Requirements (OPR), the subsequent design team Basis of Design (BOD), the Commissioning Plan, and the beginning of the Operations & Maintenance (O&M) Systems Manual. If these tasks are left until later in the process and "reverse engineered" to match the design, their usefulness as catalysts for dialog, cost and risk management, and quality tracking tools is lost."
https://www.wbdg.org/building-commissioningIt's my understanding that Commissioning begins early on in the design phase and carries through the project. The Contractor plays a part in Commissioning during construction when systems are installed and tested against the Commissioning Plan. See the link above and review the info. It helped me tremendously on CDS. Hope this helped!