How does NEC4 clause 50 work?

Clause 50 runs on assessment dates, not invoices. Under clause 50.1 the Project Manager sets the first assessment date, within the assessment interval stated in the Contract Data. Later dates fall at the end of each assessment interval until 4 weeks after the Supervisor issues the Defects Certificate. There is also an assessment at Completion of the whole of the works. Under clause 50.2 the Contractor applies before each assessment date, setting out what it thinks is due and how it worked that out. The Project Manager considers the application and gives details of how it assessed the amount due. Under clause 50.3 the amount due starts with the Price for Work Done to Date. Other amounts to be paid to the Contractor are added, and amounts to be paid by or retained from it are taken off. It is cumulative, so the payment that follows is the change since the last certificate. Clause 50.4 is the penalty for not applying. The amount due is then the lower of the Project Manager's assessment and the previous amount due. So a missed application cannot increase the payment, and may freeze it. The Price for Work Done to Date is defined differently under each main Option, which is why the same date gives different figures under Options A and C. Four mistakes are common. Contractors apply on the assessment date, when the application must be in before it. They count intervals from certificates instead of from the last assessment date. They assume assessments stop at Completion. And they add other amounts without naming the clause that makes them payable. Under the NEC4 Engineering and Construction Subcontract the same clause works one tier down. Your main contractor assesses the amount due and you apply before each assessment date, so a missed application freezes your payment in the same way.