What is update strategy transformation ?

The model you choose constitutes your update strategy, how to handle changes to existing
rows. In PowerCenter and PowerMart, you set your update strategy at two different levels:
Within a session. When you configure a session, you can instruct the Informatica Server to
either treat all rows in the same way (for example, treat all rows as inserts), or use
instructions coded into the session mapping to flag rows for different database operations.
Within a mapping. Within a mapping, you use the Update Strategy transformation to flag
rows for insert, delete, update, or reject.