Yes, it is a new category, and yes, it came about because I really didn't know how to slot these actresses who I like, but who wouldn't fit into the earlier ones. These were the actresses who made their entrance when glamour became an intrinsic part of an actress' arsenal. Nargis, Nutan, Meena Kumari, Waheeda Rehman, et al were beautiful, but they also had well-etched roles to play, strong characters who complemented their heroes, and their cinema was good. The division between 'commercial' and 'art' didn't affect them, since their world was (mostly) one of artistic commercial cinema. Madhubala, perhaps, was the one heroine of that era, who could also be considered 'glamorous'. Yet, even she had roles where her obvious beauty only complemented her character, instead of overwhelming it.
The new set of actresses were different. They were beautiful alright and, with the advent of colour, they were packaged very well. The coming of the musical love-story, the sort of entertaining fluffy confections that were different from the social commentaries that came before them, meant that cinema demanded a certain amount of polish and pizazz. And so we were introduced to a new breed of women who looked pretty, moved gracefully, and provided a perfect foil for their manly heroes. These were actresses who held audience interest by a fringe on their forehead, a dimpled smile, a coquettish look, or an upturned nose.
The roles were (mostly) interchangeable. Yet, the films they acted in wouldn't have been what they were without these actresses. Within the constraints of their circumscribed roles, they even displayed their acting chops. Some of the actresses in this category even worked with 'serious' filmmakers.