Design some kind of function, it offers interface the system which among design phases, opening it tries to send at one of development processes of the software outside (the user and the external system) vis-a-vis.

Speaking simply, on the basis of requirements specification with the process which decides "the function of the system which is developed", for example, "when A is input, in order to output B to the picture", without being conscious of mounting, you describe with user point of view. Because of this, approximately system scale becomes clear, development schedule and budget reaches the point where estimate it is possible. The user (the system order person) with SE (the designer) it is the process which is done in between.

Concretely, systems decomposition (decision of architecture), the input/output summary design (the picture/accounting table design), the code design (data format/structure), the logical data design (the logic design of the database) and the like there is a job.

Generally, the basic design/ the summarydesign it is classed, but there are times when it makes another process depending upon methodology and the scope. In addition, inserting constraint such as hardware and network in consideration, you must decide, because, including these selections, when it names the "external design", it is.