For point to point connections the a-TPInstance is used to indicate one of the endpoints. The a-TPInstance is used to represent the primary end-point of broadcast (point-to- multipoint), merge (multipoint-to-point), and composite connections. In the case of multipoint-to-multipoint connections there is no primary end point, and the a-TPInstance shall be NULL.
Sources for this description
ATM Forum M4 official network view CMIP specifications