To study product properties of digital spaces, we strongly need to formulate meaningful adjacency relations on digital products. Thus the paper \cite {H2} firstly developed a normal adjacency relation on a digital product which can play an important role in studying the multiplicative property of a digital fundamental group, and product properties of digital coverings and digitally continuous maps. The present paper mainly surveys the normal adjacency relation on a digital product, improves the assertion of Boxer and Karaca in the paper \cite{BK1} and restates Theorem 6.4 of the paper \cite{HS1}.