Hi Eileen:
It's not specific, but I would assume that the requirement is for the entire widow assembly including the mullions. Some manufacturer's have tested entire assemblies, but you're right, the majority of data is just for the glazing.
An STC 35 rating shouldn't be too hard to achieve, however, with aluminum or steel mullions. If it is a large area, packing the framing with mineral wool would help improve the STC rating.