An expression macro is a macro that you use to create repetitive or complex expressions in mappings.
You can use an expression macro to perform calculations across a set of fields or constants. For example, you might use an expression macro to replace null values in a set of fields or to label items based on a set of sales ranges.
In an expression macro, one or more input fields represent source data for the macro. An expression represents the calculations that you want to perform. And an output field represents the results of the calculations.
At run time, the task expands the expression to include all of the input fields and constants, and then writes the results to the output fields.
You can create expression macros in Expression and Aggregator transformations but you cannot combine an expression macro and an in-out parameter in an Expression transformation.