The Chan Zuckerberg Initiative, the organization that the Facebook chief executive Mark Zuckerbergand his wife, Dr. Priscilla Chan, introduced this week, has three important letters at the end of its name: L.L.C. L.L.C. stands for limited liability company, which is a specific type of business structure. The use of such an entity, revealed on Tuesday when the couple said they intended to give away 99 percent of their shares in Facebook during their lifetimes, shines a spotlight on an atypical but increasingly popular financial vehicle in philanthropy. If you are unable to follow the link, please click here for  PDF Version.