Alternate party diversion is an optional feature of telephone services, whereby a call may be routed to a different number based on time-out and precedence schemes set up by the customer.
Technical definition
Alternate party: In multilevel precedence and preemption, the call receiver, i.e. the destination user, to which a precedence call will be diverted. Diversion will occur when the response timer expires, when the call receiver is busy on a call of equal or higher precedence, or when the call receiver is busy with access resources that are non-preemptable. Alternate party diversion is an optional terminating feature that is subscribed to by the call receiver. Thus, the alternate party is specified by the call receiver at the time of subscription.
Source: From Federal Standard 1037C and from MIL-STD-188