Convert $CH_{4}$ into $CCl_{4}$ by substituting hydrogen atoms with chlorine atoms in successive reactions. Why is this reaction referred to as a substitution reaction?

(N/A) The conversion of $CH_{4}$ to $CCl_{4}$ occurs through the following successive steps in the presence of sunlight:
$1$. $CH_{4} + Cl_{2} \xrightarrow{\text{Sunlight}} CH_{3}Cl + HCl$
$2$. $CH_{3}Cl + Cl_{2} \xrightarrow{\text{Sunlight}} CH_{2}Cl_{2} + HCl$
$3$. $CH_{2}Cl_{2} + Cl_{2} \xrightarrow{\text{Sunlight}} CHCl_{3} + HCl$
$4$. $CHCl_{3} + Cl_{2} \xrightarrow{\text{Sunlight}} CCl_{4} + HCl$
This reaction is referred to as a substitution reaction because one or more hydrogen atoms of the hydrocarbon are replaced (substituted) by another atom or group of atoms (in this case,chlorine atoms).