penso significhi che la resistenza all'urto (o impatto) viene misurata indirettamente tramite la misura di una caratteristica indiretta che in qualche modo è in relazione con la caratteristica richiesta.

Forse può esserti utile guardare il sito http://www.bbri.be/homepage/index.cfm?cat=services&sub=ce&pa... del Belgian Building Research Institute, nel quale vengono illustrati i metodi accettati per la valutazione della resistenza dei materiali (anche se non si riferisce nello specifico alla resistenza all'urto):

"Assessment methods
The direct assessment by testing or calculation, where the product is subjected to a specific action after which one or more of the product characteristics are evaluated, is the preferred durability assessment method. However, indirect (proxy) testing of durability, where the product is subjected to a specific action and criteria, for which there is known to be a correlation or relationship between the characteristic being tested and the required durability characteristic is also accepted. (…) Indirect assessment, where there is no relationship between the proxy characteristic and the desired durability characteristic exist, but merely a link is also accepted."