RRGs or Relative Rotation GraphsTM (yes they are officially TradeMarked) are a visualization method to picture the relative position of all elements in a universe of securities against a benchmark and against each other. This means that RRGs can save the user time and provide additional insight which can not be achieved using regular relative strength charts.
A 'universe' can really be a collection of any sort of traded instruments/securities you like as long as their price series are comparable! Plotting an equity index in combination with a yield series in one RRG will cause problems on interpretation because of the inverted nature of a yield graph. As long as all elements of the universe are comparable RRGs can be used to analyse their relative strength.