Melting/merging 2 business objects together - a SAP standard one with the customer version. A clever concept if everybody involved in workflow development understand how this works... In practice when on a customer system a certain Business Object is used, a new object is created with its super type set to the standard SAP Business Object. E.g. BUS2012 is all about the purchase order, and a typical Z_BUS2012 would be the "customer copy". The Delegate can be filled in on a Business Object that acts as super type for another object, e.g. BUS2012 can be delegated to Z_BUS2012, merging the use of both business object definitions together.
Where? In the "Basic data" of the Business Object (Menu: Goto => Basic data) tabulator "Customizing".
Why ? in standard SAP settings, the release step of a Purchase Order can be customized - ergo via customizing settings you can make SAP throw a BUS2012 / ReleaseStepCreated event, which could then trigger your workflow(s). When the delegate for business object BUS2012 is set up, the system will also respond to Z_BUS2012 / ReleaseStepCreated.
But wait - there is more: The workflows that are delivered in standard SAP use standard Business Objects, so if it's only a few steps you would like to add, using the delegate is the way to go. On your task it may well display the BUS2012 Business Object with a Z- Method which actually lives in the Z_BUS2012 definition.
Note: the delegate is ignored when setting up starting conditions (article Event triggering)...
Note: There is only 1 delegate for a business object. Thus a blue chip company with extensive use of a business object may want to consider if all that functionality fits into "the delegate".