During the implementation of the legacy system needs to be preserved or used in the current system.
Example: A customer/ company is running business for the past 30 Years. He is maintaining customer details, vendor details, supplier details, employees and salaries, account payables, account receivables and P&L (Profit and Loss). This information is required by the customer in SAP System. So there is need to transfer the legacy system data to SAP system.
Example: Customer implemented SAP but the employees who are old cannot make use of SAP system. They would like to work on the traditional systems. After go live both systems SAP and legacy travel parallely. The data entered in the legacy system i.e. PO's, Invoices, Billing, Shipping are to be transferred periodically(Hourly) bi-hourly(for every 4 hrs) or daily
SO --- PO
BI ---- INV
Example: Reliance, Hero Honda [B2B Company to Dealers] not B2C [Distributers to customers] they communicate with dealers, suppliers, using SAP. But dealers and suppliers use non-SAP system so it is required to establish communication between SAP and NON SAP systems and perform the data transfer periodically.
ETL - Extract - Transfer - Load
SAP - Non SAP
Hero Honda XML Dealers
You can also get information regarding SAP BASIS