Just looked it up its the load rating for those tires
Since most passenger tires feature Standard or Standard Load Ranges, only Light Load, Reinforced and Extra Load tires are designated in The Tire Rack's descriptions.
Standard Euro-metric tires will have nothing branded on their sidewalls, Extra Load or Reinforced Euro-metric tires will be branded as such and identified by an "RF" or "XL" in their descriptions, as in 305/50R20 XL or 305/50R20 RF.
Standard Load P-metric tires will have nothing branded on their sidewalls or may be branded as "Standard Load," Light Load P-metric tires will be branded as "Light Load," and identified by an "LL" in their descriptions, as in P285/35R-19 LL. Additionally, Extra Load P-metric tires will be branded as "Extra Load," and identified by an "XL" in their descriptions, as in P235/75R-15 XL.
Only P-Metric sized tires with 45-series aspect ratios or lower may be manufactured in a Light Load configuration. Light Load tires are designed to carry less weight than Standard Load tires and have been developed for specific applications (typically when relatively large tire sizes are used as Original Equipment (O.E.) on relatively small cars or for Track & Competition DOT tires used for racing applications).
In all cases, when changing tire sizes or converting from Euro-metric sizes to P-metric sizes, it is important to confirm that the Load Index in the tire's Service Description of the new tire is equal to or greater than the Load Index of the original tire.