Press cloths pick up sticky residue from fusible interfacing. Always having the same side of a press cloth towards the interfacing ensures that your iron’s sole plate won’t pick up any of that gunk. I sew the word “UP” into my press cloths to mark the top side. “UP” is only two-letter long whereas “TOP” requires 50% more exertion! I suppose I could have used a meaningless (but functional) “G”.