Passage: When he began his studies of Mỹ Sơn in 1899, Henri Parmentier found the remnants of 71 temples. He classified them into 14 groups, including 10 principal groups each consisting of multiple temples. For purposes of identification, he assigned a letter to each of these principal groups: A, A', B, C, D, E, F, G, H, K. Within each group, he assigned numbers to the edifices comprising it. Thus "My Son E1" refers to the edifice at My Son belonging to group "E" that has been assigned the number "1."

Question: Why did he designate letters to the groups?
For purposes of identification