Imagine we have a steel rod with a $1\mathrm{.}1\%$ carbon content. This rod undergoes decarburization during annealing in air at $950\backslash$deg C$.$ In this situation, in the formula;

$\frac{C-Cs}{C0-Cs}=$erf$(\frac{z}{2\sqrt[\phantom{2}]{Dt}})$

should we take the $"$C$0"$ as $0,$ or the $"$Cs$"$ as $0?$