Commercial tobacco is tested for a variety of components after it has been redried. This is usually done by taking samples of several bales of the same lot produced by either a larger grower or co-op (which comprises of several to many smaller producers located within a region). The testing for Flue Cured tobacco determines the color, sugar and nicotine. As a general rule, the higher the sugar, the more value the "lot" has. The sugar contents is what makes the tobacco "smooth", but it is most often used as a guide for the Master Blender to recreate a particular brand.
For example....if the Marlboro Reds blends have a sugar content of 10% (I'm pulling these numbers out my of arse), the Master Blender would then know how much 20% to blend with 4% to achieve the desired result. When blending, consideration also needs to be given to the nicotine content as well as protein and other components in other types of tobacco.
So, when somebody makes a claim of "Sweet", the smooth almost is inherent, but this claim is very much verifiable and should not be used merely as a marketing ploy. When I claim my FC is "sweet and smooth" (yes, they copied my wording verbatim), I can prove my claim is true. The way the competition buys their tobacco, I'm not sure if they can do the same.