Clause 11 requires that the Works shall be in the condition required by the Contract at the end of the Defects Notification Period. Where the Contractor carries out work in the Defects Notification Period, it is not entitled to receive payment if the work was a result of a defect in the design for which the Contractor was responsible. Similarly, if the Plant, Materials or workmanship are not in accordance with the Contract or there is a failure by the Contractor to comply with any other obligation then it is required to remedy the problem without payment. The Employer may obtain an extension of the Defects Notification Period if the Works, a Section or a major piece of Plant cannot be used during the Defects Notification Period. The Contractor is required to remedy any defect during the Defect Notification Period and, if it does not, the Employer may claim against the Contractor. Rights are given to the Contractor to undertake this work subject to the Employer’s reasonable security restrictions. Once the Defects Notification Period has expired the Engineer is required within 28 days, subject to receipt of the Contractor’s Documents and the completion of any tests, to issue a Performance Certificate. It is the Performance Certificate that is deemed to constitute acceptance of the Works. Sub-Clause 11.10 provides that after the Performance Certificate has been issued, each Party will remain liable for the fulfilment of any obligation which remains unperformed at the time. The extent and meaning of this clause is open to debate.