Akshar Purushottam Upasana is an interpretation by Shastriji Maharaj of the philosophy set forth by Swaminarayan which led to the creation of the Bochasanwasi Shri Akshar Purushottam Swaminarayan Sanstha (BAPS) in 1907. It could also be referred to as Brahman Parabrahman Upasana or Bhakta Bhagwan Upasana. According to this interpretation, Gunatitanand Swami is the successor of Swaminarayan as opposed to the Acharyas (heads of the Swaminarayan Sampraday). Shastri Yagnapurushdas created BAPS as a result of this difference in belief.
Read more about Akshar Purushottam Upasana: Interpretation of Bhagwan Swaminarayan's Teachings, Gunatitanand Swami As Mul Akshar, Gunatitanand Swami As Successor