Difference among start routines, end routines and expert routines

Start Routine: Start routine runs before the transformation rules. It manipulates the source data package. The source data package is in the structure of data source. The start routine has a table in the format of the source structure as input and output parameters. It is used to perform preliminary calculations and store these in a global data structure or in a table. This structure or table can be accessed from other routines. You can modify or delete data in the data package. Generally used for Filtering records.

End Routine: End routine runs after the transformation rules. It manipulates the target data package. The result package is in the structure of the target object. It is a routine with a table in the target structure format as input and output parameters. You can use an end routine to post process data after transformation on a package-by-package basis. For example, you can delete records that are not to be updated, or perform data checks. 

Expert Routine: This will trigger without any transformation Rule. Whenever we try to write a expert routine, all existing rules are deleted. This is used generally for customizing rules. It is helpful if complex or better performing transformations are needed.