Can assimilated energy be simply lost from a trophic level?
In this diagram, since this is the only loss between trophic levels (along with  respiration and NA within each trophic level), NU represents dead organic matter, that is not transferred along the food chain. In other words, this represents dead leaves that are not eaten by herbivores, hares that die of old age and not predation, and foxes that are killed by falling rocks. So NU is the amount of energy that goes directly into the  decomposer community (another flow to decomposers is coming from matter that is excreted by herbivores and predators), since this diagram only includes primary producers, herbivores and predators.

NA, on the other hand, represents energy that is consumed by the next trophic level but not assimilated (Not Aassimilated). So this represents faeces etc.