Direct Transfer Trips (DTT) are initiated from station relays when a severe event occurs in the substation. Some of these events are breaker failure, bus faults, transformer failure, etc. A lockout relay (86 device) is assigned to each event.
The lockout relay in the station is pretty essential. When operated due to any of the events indicated above, it trips all the breakers connected to it. So for example, for a bus fault, the bus differential relay will operate the bus lockout relay which will trip and blocks re-closing of all the breakers connected to the bus. If the breaker is at a remote location and can’t be physically wired, a DTT signal is sent to it via the protective relays using the pilot channel.
The figure below shows which all lockout relays are typically used to initiate the DTT. The substation design affects this. For example, a ring bus substation or a breaker-and-a-half substation has a breaker failure lockout assigned to each breaker. For other designs, a breaker failure event trips the bus lockout relay. Whichever case you are presented with, a contact from the lockout is assigned to the relay that can key the DTT signal to the remote end breaker.
Note that lockout relays are typically used to initiate DTT. In which case, the intent is to trip all breakers without any delay. There is no logic or reasoning – just trip. Remaining events like faults on lines are taken care of by the pilot schemes which have intentional delay for coordinating the breaker trips.