What does NEC4 clause 60.1(17) mean?

In the NEC4 main contract the assumption is the Project Manager's. On an NEC subcontract it is your main contractor's. This is the only route by which a settled compensation event is revisited. When an event's effects are too uncertain to forecast, the other side can state assumptions about them. The quotation is then assessed on those assumptions. If an assumption later proves wrong, it notifies a correction, and that correction is a new compensation event. The event is the notified correction, not the discovery that a forecast was wrong. And the assumption must be one the other side stated, not one you made. Your own assumption in your quotation does not count. Under clause 66.2 a settled assessment is not revised, so a wrong forecast of yours stays wrong. To rely on the clause, show the earlier event, the assumption stated about it in writing, and the notice correcting it. If you think an assumption is wrong, ask in writing for it to be corrected. If that is refused, there is no event. Your main contractor should notify the event with the correction and ask you to quote. If it does not, you notify under clause 61.3, and the 8-week bar does not apply. The change to the Prices, your contract price, is assessed under clause 63.1 as the difference the correction makes, plus the Fee. It can reduce the Prices as well as increase them. What goes wrong most is that assumptions are never stated in writing. The later argument is then about whether one was stated at all. Any delay to the Completion Date is assessed separately under clause 63, and that is a job for your planner.