我wiki見到咁寫"Predating the term “rank”, sometimes also called “row” is the use of single-sided and double-sided modules, especially with SIMMs. While most often the number of sides used to carry RAM chips corresponded to the number of ranks, sometimes they did not. This could lead to confusion and technical issues."
rank可能指單面/雙面
https://en.wikipedia.org/wiki/Memory_rank