One application and a simplistic network

In the 1960s, the z/TPF system was designed for the airline reservation application, the only application in a z/TPF system. Therefore, by default, all terminals in a network attached to a z/TPF processing facility were logged onto the only application (the airline reservation application). Also, at that time, the technology of terminal networks was rather simplistic, with communication controllers providing little more function than line multiplexing. As such, the z/TPF system (also called a host system) was responsible for:

Requesting input from the terminal interchanges in the network, also called polling.
In a network where communication controllers provide only minimal function, the host system (the z/TPF system in this case) must check whether data has been input from terminals. Contrast this with a modern communication controller that performs this function on behalf of the host to which it is attached.
Scheduling output (that is, responses) to those same terminals.
In a network where communication controllers provide only minimal function, if a device is not ready to receive data, the data (that is, a response) must be queued by the host system (that is, the z/TPF system) until the device is ready to receive it. Compare this to a modern communication controller that takes the data from the host and buffers it until the device is ready to receive it.

An application program addressed terminals by physical addresses. The z/TPF system handled the unique interfaces that were required for each different device type (similar to a communication protocol). The application recognized a terminal by its terminal address in the form of line number (LN), interchange address (IA), and terminal address (TA) or LNIATA.