Vision document
A Vision Document is a software process document that describes the overall ‘vision’, or plan, for a particular piece of software. It defines the stakeholders view of the product to be developed, specified in terms of the stakeholders key needs and features. Containing an outline of the envisioned core requirements, it provides the contractual basis for the more detailed technical requirements. It is much shorter and more general than a product requirements document or a marketing requirements document, which outline the specific product plan and marketing plan respectively.
Purpose
The Vision provides a high-level, sometimes contractual, basis for the more detailed technical requirements. It captures the “essence” of the envisaged solution in the form of high-level requirements and design constraints that give the reader an overview of the system to be developed from a behavioral requirements perspective. It provides input to the project-approval process and is, therefore, closely related to the Business case. It communicates the fundamental “why and what” for the project and is a gauge against which all future decisions should be validated.