This selects how the sampled variable's value is turned into the number compared against the thresholds: absoluteValue uses the raw reading at the end of the interval, while deltaValue subtracts the prior sample from the current one to get a rate of change. Choosing deltaValue is what lets an admin alarm on how much a counter increased in the last interval rather than on its raw cumulative value, which matters for ever-incrementing counters like error or packet counts. For a rapidly climbing interface error counter, an admin would pick deltaValue so the alarm fires when errors-per-interval spikes, not just when the lifetime total crosses some large number.