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Anything you keep for future reference. It is very important not to blur the lines between things that require action and those that are purely reference material.
The practice of keeping strict boundaries between items of different types. In GTD, for example, it is highly recommended to keep reference material (which has no emotional cost) separate from commitments and actions you need to keep (which have strong emotional pull). If you blur the edges between items of different types, your system will not be effective.