What does NEC4 clause 60.1(17) mean?

Clause 60.1(17) is the only route by which an implemented compensation event is revisited. When the effects of a compensation event are too uncertain to forecast reasonably, the Project Manager may state assumptions about them and the quotation is assessed on those assumptions; when an assumption later proves wrong, the Project Manager notifies a correction, and clause 60.1(17) makes that notification a new compensation event. The event is not the discovery that a forecast was wrong, but the notified correction to an assumption the Project Manager, not the Contractor, stated. A Contractor's own assumption in its quotation is not one the Project Manager stated, and under clause 66.2 an implemented assessment is not revised, so a wrong Contractor forecast stays wrong. To rely on the paragraph the Contractor shows the earlier compensation event, the assumption the Project Manager stated in writing about it, and the Project Manager's notification correcting it. Where the Contractor believes an assumption is wrong, it asks the Project Manager in writing to correct it; if the Project Manager refuses there is no event. Because the event arises from the Project Manager's own notification, the Project Manager notifies the compensation event with the correction and instructs a quotation; if it does not, the Contractor notifies under clause 61.3 without the eight week bar, because the Project Manager should have notified. The change to the Prices is assessed under clause 63.1 as the effect of the correction on actual Defined Cost by the date of the notification and forecast Defined Cost afterwards, plus the Fee, which is the difference the correction makes, and it can reduce the Prices as well as increase them. What most often goes wrong is that the Project Manager never records assumptions in writing, so the later dispute is about whether an assumption was stated at all. Any delay to the Completion Date is assessed separately under clause 63 by the programming expert.