Global variables are used for mapping source elements, mapping functions or constants to target elements. They are declared and used globally for all target elements.
To use a global variable in a mapping activity, you need to map it to a target element. This will successfully execute the global variable when you execute the mapping activity in the process flow.
A global variable differs from a context variable, in the respect that it is available only within the selected mapping activity. Context variables are available in the process designer too.
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Once you have created a global variable, you can map it to a target element.