Generally, it is used when refering to someone who posseses or aspires (painfully) to the stereotypical characteristics of what is seen as a higher social class (in the traditional, fixed sense).
For example, in a wealthy public school a rah may have to envoke an image of old aristocracy and do so in a pompous and superior manner - usually demonstrated in an expensive, flamboyant lifestyle and often loudly declared opinion.
Whereas in an enivronment where there is fewer generally exuberant lifestyles an individual could be termed a rah simply for being obviously more asperational or often just wealthy in itself.
Has many connotations of succeeding only as a result of old boy networks and from being fully supported by parents.
Sebastian of Bridehead Revisited could be described as a rah.