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Variables can be used in many steps, mostly there is a "Variable..." button present next to the field.
The following 2 screenshots show the transformation and step that uses the variable we defined earlier:
As you can see in this transformation we collect the processed file names and send them to the next job entry.
Also, if you would run or preview this transformation in Spoon, a dialog will pop up to allow you to enter a value for TODAY.
This way you can test the transformation without running the complete job.
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