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Something you create (and trust) to alert you when you are required to do (or remember) something.
Anything that has a day or time context to it should be placed on your calendar.
Any input that has arrived in your world for which you have not defined the meaning.
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.