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For knowledge workers, there is a lot of work to do defining our work before we can start actually doing our work. The 5 phases of workflow is the GTD model for capturing, defining and doing our work.
Defines explicitly what we naturally do when planning anything. David Allen recommends we use it consciously to increase personal and team productivity. If more clarity is required on a project, move up the model. If more action is required, move down the model.