Reusable transformations can be used in multiple mappings.When you need to incorporate this transformation into maping, U add an instance of it to maping.Later if you change the definition of the transformation ,all instances of it inherit the changes.Since the instance of reusable transforamation is a pointer to that transforamtion, You can change the transforamation in the transformation developer,its instances automatically reflect these changes.This feature can save you great deal of work.

A reusable Transformation is a reusable metadata object , defined with business logic using single Transformation.