Tennis racket stiffness (RA) explained

When available, we always mention the RA value in the racket specs. This represents the stiffness of a racket. A small manual in this: the stiffness of a tennis racket (RA) explained . And what are the advantages of both flexible and stiff rackets and which RA corresponds to which? Low: 60 or lower Medium: 61 to 68 High: 69 and above