Diamond Comic Distributors used the top selling Batman issue each month as the benchmark against which all other comics were measured. An "index" value of 100 meant equal sales to Batman; 50 meant half as many copies sold.
This system originated with Capital City Distribution in the 1980s (originally using Uncanny X Men as the benchmark). When Diamond became the dominant distributor, they switched to Batman — a reflection of the character's remarkable consistency as a top seller across every era.