Update Strategy Transformation in Informatica , is a connected and active transformation.Update Strategy transformation let you  insert, update , delete data from target Data also. It can also reject the incoming data depending upon your condition.For example : when a customer is changing address , you can use this update strategy transformation to update data in your Customer Tables. In below article we will go through the properties of Update Strategy Transformation. We will also discuss the steps of adding /configuring Update Strategy  transformation in Informatica Mapping You can define update operation on below two levels:

Update Strategy on  Session level:

We can instruct Informatica Service to treat all rows in the same way or use same instruction by configuring Target mapping at session level. Update Strategy on  Mapping level: You can flag a row (for insert , update, delete or Reject depending upon some condition) by adding "Update strategy Expression" in the properties tab of Update Strategy Transformation in a Mapping. Below Flags can be set in Update strategy Expression.

Flag	Numeric Value	Type Operation
DD_INSERT	0	Insert row
DD_UPDATE	1	Update row
DD_DELETE	2	Delete row
DD_REJECT	3	Reject row

For more details on Update Transformation , Please check here A well explained example can be found here as well.