In short, sectional completion is an obligation to meet the date for completion for just part of the works. Whereas partial possession is an agreement to accept completion for just part of the works. It is usually agreed whilst the works are progressing.
Sectional Completion and the consequences are agreed up front and provided for in the contract. Once it is in the contract, the contractor has an obligation to achieve the sectional completion date. Liquidated damages are agreed up front should the section be delivered late.
Requires an agreement during the progress of the works. It can be more disruptive for the contractor if it is not planned well in advance.
The contractor must act reasonably and not delay in reaching his decision. He can refuse it if it will interrupt the progress of the works; make the balance of the works more expensive to complete; cause real insurance issues or pose a health and safety risk. The employer can pay these costs to obtain partial possession, but it will often be more expensive than agreeing sectional completion at the outset of a contract.
It is necessary to reach agreement on the liquidated damages to be paid by the contractor if the balance of the works are not completed on time.