Z-scores are used to standardize series and enable comparison mortality pattern between different populations or between different time periods. The standard deviation is the unit of measurement of the z-score. It allows comparison of observations from different normal distributions.
In general, Z-score = (x-mean of the population)/Standard deviation of the population, which could be approximated in our context by S-score = (number of deaths - baseline) / Standard deviation of the residuals (variation of the number of deaths around the baseline) on the part of the series used to fit the model, used as the standard unit.
Z-score are computed on the de-trended and de-seasonalized series, after a 2/3 powers transformation according to the method described in Farrington et al. 1996. This enables the computation of Z-scores for series that are originally Poisson distributed.