Is it the flow diagram, User Interface or what?
In the design phase of the SDLC, making solution is focused via system modeling. Uml modeling is performed which is called the Unified modeling language. UML modeling has the following steps
The UML modeling is basically for defining the problem or we can say it is a tool to solve the problem.
More : http://websolace.net/web-design-solutions/sdlc-maintains-the-quality/