The terms 900 GSM, 40×40, and 40×60 are used to describe different characteristics of microfiber towels. GSM stands for grams per square meter and is a measurement of the density or thickness of a fabric. In the case of microfiber towels, a higher GSM typically means a thicker and more absorbent towel. So, a 900 GSM microfiber towel would be very thick and absorbent.The numbers 40×40 and 40×60 refer to the size of the individual fibers that make up the towel. Specifically, they refer to the number of fibers per square inch of the towel. In this case, a 40×40 microfiber towel would have 1,600 fibers per square inch, while a 40×60 microfiber towel would have 2,400 fibers per square inch. The higher the number of fibers per square inch, the softer and more gentle the towel will be on delicate surfaces.