(1) When operations as viewed from outside the system are the same, regardless of the internal structure, it is desirable to first write the descriptions that are independent of internal structure. (It is common for exchanges to have various architectures, but to have completely identical function as seen from the outside.)
(2 ) At the stage of description from an external point of view, including operations at the time of service composition, it is desirable to define operations formally.
(3) At the stage of operations described from an external point of view, it is desirable to determine the logical conflicts of each service, and support operations design according to the circumstances in which the services are combined.
(4) After specifying the operations which are independent of internal structure, it is desirable to describe the details of operations which do depend on internal structure, with the aid of a computer.
In STR, the telecommunication system in question is regarded in the following way so as to take the internal structure of the network as a black box. "Within a telecommunications system, there are an unspecified number of terminal control processes. These terminal control processes all have identical functions. There are no processes other than terminal processes."
The STR description method specifies the range of influence for when one event occurs, and all of the state transitions among terminals that are included in that range are described.