What does NEC4 clause 60.1(21) mean?

Clause 60.1(21) lets the Client extend the list. The event is an additional compensation event stated in the Contract Data. The Client completes the Contract Data before tender, and one of its entries allows the Client to state additional compensation events; anything written there becomes a compensation event with the same status as the twenty numbered paragraphs, without any need for a Z clause. Typical additions are risks the Client is better placed to carry, such as delay by a statutory undertaker beyond a stated date or contamination beyond a stated level, but the wording is whatever the Client wrote. An event drafted loosely is generous, one drafted with conditions is only as wide as those conditions, and a blank entry means the paragraph adds nothing. To rely on the paragraph the Contractor shows the Contract Data entry, that the event which happened is the event described there, and that any conditions in the entry are met. The Contractor notifies the event under clause 61.3 within eight weeks of becoming aware that it has happened, unless the entry describes an event arising from a Project Manager's instruction or decision, in which case the Project Manager notifies it. The change to the Prices is assessed under clause 63.1 as the effect on actual Defined Cost by the date of the notification and forecast Defined Cost afterwards, plus the Fee, exactly as for the numbered events, and the Project Manager's decision under clause 61.4 applies in the same way. What most often goes wrong is that the Contractor assumes the standard list is the whole list and never reads the Contract Data, so an event the Client had expressly accepted is never notified, or that an additional event overlaps a numbered paragraph and the Contractor notifies under the wrong one. Any delay to the Completion Date is assessed separately under clause 63 by the programming expert.