Variables can be used in most text properties for Actions. To use a Variable in a property, enclose the variable in percent symbols,
eg. : %OUTPUTDIR%
If you need to use a % symbol in a property, make sure you escape it with another %, eg. %%. See Escaping Variable Contents.
A common use for variables is to set file paths, for example to set the File Spec for Set File Attributes:
Variables can also be used in Script Events.
See also: Variable Sense.