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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.
A mechanism to remind yourself of something at a given time in the future. When the time arrives, the contents of the tickler file are moved into an inbox for processing with all other stuff.