How does NEC4 clause 50 work?

Clause 50 is the assessment, and it runs on assessment dates rather than invoices. Under clause 50.1 the Project Manager assesses the amount due at each assessment date. The first assessment date is decided by the Project Manager within the assessment interval stated in the Contract Data; later assessment dates occur at the end of each assessment interval until four weeks after the Supervisor issues the Defects Certificate, and there is also an assessment at Completion of the whole of the works. Under clause 50.2 the amount due is the Price for Work Done to Date, plus other amounts to be paid to the Contractor, less amounts to be paid by or retained from the Contractor. The amount due is cumulative, so the payment that follows is the change since the last certificate. Under clause 50.3 the Contractor submits an application for payment before each assessment date, setting out the amount it considers due and how it has been assessed, and the Project Manager considers it and gives the Contractor details of how the amount due has been assessed. Clause 50.4 is the penalty for not applying: where no application is made, the amount due is the lesser of the amount the Project Manager assesses and the amount due at the previous assessment date, so a missed application cannot increase the payment and may freeze it. The Price for Work Done to Date is defined differently for each main option, which is why the same assessment date produces different arithmetic under Options A and C. What goes wrong: Contractors treat the assessment date as the day to apply rather than the day by which the application must be in; intervals are counted from certificates instead of from the previous assessment date; assessments are assumed to stop at Completion; and other amounts in both directions go in without the contract provision that makes them payable being identified.