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This page covers the differences between a vision statement and a mission statement, and in what circumstances you might use them.
A vision statement is a forward looking statement defining the aims, focus areas and goals of a project.
A mission statement is a statement of what work part of your current operation needs, but unlike a vision it does not need to consider the future.
Providing the team with a vision or goals for the project they are working on gives them an understanding of the aims of the project. A vision also provides them a framework to help prioritize their work on the basis of how close a particular task or feature is in meeting the vision or goals of the project.
For new projects where you have long term goals, providing your team with a vision statement helps them to be forward-looking in their planning and understand how their current works fits into the long-term goals.
Legacy projects are better defined using mission statements. This ensures that the team and your users understand what is on task for your project and helps to enable your team to make the legacy project sustainable and scaleable.