This is an Interfit 39" (99 cm), Silver Umbrella. Silver umbrellas are used to bounce light onto a subject. They provide softening, with a harder shadow than a regular white umbrella. Light loss is usually ~1.5 f/stop compared to the same light used direct, bare light. The black backing cuts down on stray light.
Use this size umbrella for a 1 - 2 person half-length portrait.