J2 and JR refer to the material’s sub-grade - its fracture toughness - which is measured in joules [J] of a material’s toughness at a particular temperature.
J2 is a material with a minimum of 27 joules at -20C while JR is a material with a minimum of 27 Joules at +20C (room temperature).
A JR material is normally used in an internal environment whereas a J2 is for external use.
If J2 is required you must not substitute JR.