Parti refers to a tri color yorkie. Many believe Biewers are tri color yorkies that are from a specific German line, originated by a man named, Werner Biewer. Some Biewer owners are trying to get the Biewer recognized as a different breed and therefore do not call them yorkies.
Below are several threads that you may want to read. If this isn't enough info, there are numerous threads on YT discussing this subject.
http://www.yorkietalk.com/forums/gen...ti-yorkie.html http://www.yorkietalk.com/forums/gen...ou-prefer.html http://www.yorkietalk.com/forums/gen...kes-parti.html http://www.yorkietalk.com/forums/gen...ti-beiwer.html