The so-called carbon equivalent of austenite liquidus (CEL) and eutectic carbon equivalent (CE) that are used in cast iron metallurgy are known as linear functions of the composition. This paper first reminds how CEL and CE expressions are obtained from the knowledge of the relevant equilibrium phase diagram, emphasizing that they are not the same quantities. To account for the observed differences between these expressions and the experimental ones, it has been proposed in the literature to add kinetic terms to the equilibrium expressions. This is first discussed by considering austenite liquidus data for cast irons with 1 wt.% to 3 wt.% silicon, from which the appropriateness of the CEL is confirmed while also evidencing the role of austenite growth undercooling. Then, experimental results on the solidification onset of near eutectic (slightly hypo-and mildly hyper-eutectic) and strongly hyper-eutectic cast irons are considered for discussing the significance of CE. For this second part, alloys that solidify in the stable system were considered whose results show again the role of austenite growth undercooling, but also that of graphite growth undercooling. The effects of addition of magnesium and of inoculation are discussed.