SAP BAPI : Business Application programming interfaces are standard methods to access business objects which form the core of all SAP R/3 business frameworks.
There is two main methods to find a BAPI:
Find SAP BAPI – Method 1
Go to BAPI transaction
Select the busines object you want a SAP BAPI for ( for example SalesOrder )
Method 2 to Find BAPI in SAP SD
In you know the BUS of a business object, you can navigate directly to the the last screen,
Go to SE38, search for *BUSXXX, you will launch the “Display Object Type”