This attribute represents the association between a multipoint connection and the underlying termination points. For point to point connections the a single entry in the z- TPList is used to indicate one of the endpoints. Multiple entries in the z-TPList are used to represent the secondary end points of broadcast (point-to-multipoint), merge (multipoint-to- point), and composite connections. The a-TPInstance identifies the primary endpoint. Only the z-TPList is used identify all end points in a multipoint-to-multipoint connection (there is no primary end point).
Sources for this description
ATM Forum M4 official network view CMIP specifications