3 month Relative Strength measures a stock's price change over the last month relative to the price change of a market index. In the case of UK stocks, this is the FTSE All-share. US stocks are compared to the S&P 500. EU stocks are compared against the FTSEurofirst 300. It shows the relative outperformance or underperformance of the stock in that timeframe.
This value is sourced from Thomson Reuters. It is calculated dividing the price change of a stock by the price change of the index for the same time period. e.g. A stock falling by 20% versus an index rising 20% would lead to a Relative strength calculation of 100 * ( 80/120 - 1) = -33%