The Lineage
Lineage is one of the most important concepts for Tibetan Buddhists, because it shows a continuity of the unbroken transmission of the complete teachings from master to student that can be traced back to the Buddha Sakyamuni himself.
When the master passes away the student has grown into master, and they in-turn find the reincarnation, and pass the complete teachings back.
The central holders of the teachings become part of the official lineage upon passing away, but all Rinpoches and Tulkus are lineage holders in that they have received some transmission of the teachings which they transfer to their students.