Aggregator transformations are handled in chunks of instructions during each run. It stores transitional values which are found in local buffer memory. Aggregators provides extra cache files for storing the transformation values if extra memory is required.
Can one use mapping parameter or variables created in one mapping into any other reusable transformation?
Yes, One can do because reusable transformation does not contain any mapplet or mapping.
It is used for looking up data in a relational table through mapping. Lookup definition from any relational database is imported from a source which has tendency of connecting client and server. One can use multiple lookup transformation in a mapping.