Umbrellas soften, broaden, and diminish the light output of any tungsten or flash light source. Umbrellas with a white interior will soften and weaken light more than an umbrella with a silver lining. A white umbrella without a black backing may also be used as a makeshift softbox, although the use of the light will not be as efficient as with a lightbox.
Choosing umbrella size is determined by the size of the subject, and the strength of the light supply being used. Choosing an umbrella surface is as subjective as choosing a paint and brush.
Triple-use umbrella is translucent white, with removeable silver backing.
Use the translucent white as a sunlight diffuser, or with a strobe as a makeshift softbox.
Use the silver backing to add a slight specular highlight when bouncing the light output of a strobe, or small tungsten-halogen light head.