Boost smart pointers.

Hello All, I have been trying to understand the smart pointer library. One thing that I did not understand was in the documentationin quite beginning: "T may be an incomplete type at the point of smart pointer declaration. Unless otherwise specified, it is required that T be a complete type at points of smart pointer instantiation." So what exactly does this incomplete/complete type mean? Does this have anything to do with the class pointed to being abstract /concrete ? thanks, regards, amit.