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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.
Something you need to know on a given day, such as the directions to the seminar you're attending.
A system of classifying into files (usually arranged alphabetically)
Anything you're committed to that will take more than one action step to complete.
Any input that has arrived in your world for which you have not defined the meaning.