I don't think it refers to a sigma chart...It came from a bad Japanese to English translation; here it is in context:
"X(-bar)-Rs is used for the same purpose as the Xbar -R Control Chart, but used for control with a piece of sample. When plural samplings are difficult in the group (e.g., destructive testing, much time needed for measurement, etc.) X(-bar)- Rs Control Chart is used."
And then later: "How many data is in the group? One => Use X(-bar)-Rs Control Chart."
I feel like it is sort of referring to an I-MR chart, but I'm no SPC expert, so I was wondering if an X(-bar)-Rs chart is one I haven't heard of, or if it's an old way of referring to an I-MR chart...