What is the difference between a court-appointed or volunteer amicus curiae in a custody case?

Olive Stone, The Child’s Voice in the Court of Law (1982), at p. 155 describes the difference: “In Beard v. Travers, as in most of the other circumstances in which an amicus curiae is given leave to intervene, he is a volunteer who applies to the court and is granted leave to appear and represent some third-party interest … In the current practice in Alberta, the amicus curiae in children’s custody cases is appointed by the court, instructed by the court, responsible to the court, and either is or should be paid either from public funds or from funds provided by the child’s parent(s).” (underlining is mine)