Custom transformations operate in conjunction with procedures you create outside of the Designer to extend PowerCenter functionality. The Custom transformation functions allow you to develop the transformation logic in a procedure you associate with a Custom transformation. PowerCenter provides two sets of functions called generated and API functions. The Integration Service uses generated functions to interface with the procedure. When you create a Custom transformation and generate the source code files, the Designer includes the generated functions in the files. Use the API functions in the procedure code to develop the transformation logic.
When you write the procedure code, you can configure it to receive a block of rows from the Integration Service or a single row at a time. You can increase the procedure performance when it receives and processes a block of rows.