Lets check some concepts, scenarios and tips related to BI/BW from ABAP end. 

In BI/BW ABAP might be required in a start routine, end routine, expert routine, InfoPackage, Data Transfer Process, Analysis Process Designer, query, or field.

KEY CONCEPT:

Start routine is a routine in transformation that is executed before transformation is executed. End routine is a routine in transformation that is executed after transformation is executed. Expert routine is a routine in transformation that is itself the transformation. In other words it contains all three: start, end, and actual. SOURCE_PACKAGE is a structure that contains the inbound fields of the routine. RESULT_PACKAGE is a structure that contains the outbound fields of the routine. APD is a workbench with a graphical user interface (UI) for creating, executing, and monitoring analysis processes.SAP NetWeaver BW provides user exits throughout the system to take advantage of custom coding add-ons and to be able to meet user requirements.

START ROUTINE :

A start routine is used in the transformation when fetching information from other BW objects is necessary, or when we need to perform calculations or other data transformations and then store them in a global data structure or table before transformation is triggered. The start routine is run for each data package at the start of the transformation.