Consists in conceiving the system’s architecture, user interfaces, and user experiences to fulfill the defined requirements.
Design Documents and Request for Comments (RFCs) are tools used by engineering teams to plan non-trivial projects. They are great tools to provide a clear specification and help clarifying the project vision, direction and assumptions across the organization.
They facilitate communication and feedback, create documentation, among other benefits.
They also work pretty well for asynchronous teams.
A few references: