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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.
The next defined physical, visible action you will take to advance some purpose.