Software design documents explain how a specific piece of software or software feature should be developed. They're important technical documents that focus on the how of the development process.
Statement of work
A statement of work, also known as a SoW, is an essential project planning and project management tool. At its most basic, it acts as a kind of formative contract between a customer and a company.
You might be wondering what technical documentation is, but it's difficult to describe. It's an umbrella term that refers to a wide range of technical documents that explain the use, functionality or development of a given product.