Here is a scholarly work by Sadguru Shri Muni Swami Shri Keshavpriyadasji explaining the subtle distinction between Avtar and Avtari -Incarnation and the Godhead of Incarnations. The knowledge of this essential distinction between the two i. e. the cause and effect will enable the devotees to worship God with proper understanding 'Upasana' and "Nishchay1 - firm faith in God.