How does NEC4 clause 63 work?

Clause 63 sets the rules every quotation and every own assessment must follow, and clause 63.1 is the core. The Prices are your contract price, Defined Cost is the contract's measure of cost, and the Fee is its percentage for overheads and profit. Where the event came from an instruction or changed decision, the dividing date is the date of that communication. Otherwise it is the date of the notification. Everything before that date is assessed from records. Everything after it is a forecast. That split is what makes the assessment a forecast rather than a claim. Clause 63 also requires cost and time risk allowances for matters that have a significant chance of happening and are your risk under the contract. It assumes you reacted competently and promptly. Under Options A and B, both sides can agree to use rates and lump sums instead of Defined Cost and the Fee, but only by agreement. Delay to the Completion Date is assessed separately under clause 63, from the effect on planned Completion shown on the Accepted Programme. That is programme work for your planner. Four things go wrong. Quotations are priced from bill rates or invoices, with no link to the Schedule of Cost Components. There is no split at the dividing date, so actual and forecast cost are muddled. The Fee is left off, or applied to the wrong base. And there is no risk allowance, so the forecast is optimistic. Once the event is implemented under clause 66, that cannot be corrected when the risk happens.