In the case of PPR :

D₁₀₀ = the minimum dose for which the photoresist will completely dissolve (harden in case of NPR) when developed. D₀ as the maximum energy density for which the photoresist will not dissolve (harden in case of NPR) at all when developed.

Between these values, the photoresist will partially dissolve (harden in case of NPR).

For PPR, the contrast is related to the rate of polymer chain scission (breakings) and the rate of change of solubility that is solvent dependent. For NPR, the contrast is related to the rate of polymer chain formation.