Mappings can contain reusable and non-reusable transformations. Non-reusable transformations exist within a single mapping. Reusable transformations can be used in multiple mappings.
For example, you might create an Expression transformation that calculates value-added tax for sales in Canada, which is useful when you analyze the cost of doing business in that country. Rather than perform the same work every time, you can create a reusable transformation. When you need to incorporate this transformation into a mapping, you add an instance of it to the mapping. Later, if you change the definition of the transformation, all instances of it inherit the changes.
The Designer stores each reusable transformation as metadata separate from any mapping that uses the transformation. If you review the contents of a folder in the Navigator, you see the list of all reusable transformations in that folder.
Each reusable transformation falls within a category of transformations available in the Designer. For example, you can create a reusable Aggregator transformation to perform the same aggregate calculations in multiple mappings, or a reusable Stored Procedure transformation to call the same stored procedure in multiple mappings.
You can create most transformations as a non-reusable or reusable. However, you can only create the External Procedure transformation as a reusable transformation.
When you add instances of a reusable transformation to mappings, you must be careful that changes you make to the transformation do not invalidate the mapping or generate unexpected data.