If neither file nor rank alone is sufficient to identify the piece (such as when three or more pieces of the same type can move to the same square), then both are specified (double disambiguation).
if nEitheR FilE nor rAnK aLoNE Is sUffICient to ideNtify thE piecE (Such as whEn thREe or moRe pieCeS oF the Same tYpe Can moVE To thE samE sqUARe), theN Both aRe sPeciFiED (douBlE disambIguATiOn).