SAP implementations are done in real life with a lot of considerations. One consideration is integration with other SAP & non-SAP systems. For many reasons, customers have to use many SAP & non-SAP systems and during implementation, the implementation team has to consider these scenarios carefully.
Non-SAP systems are used for many reasons. For example many hardware facing applications must be integrated with the SAP system. Hardware systems like bar code readers, RFID etc. which are used extensively in warehouses and retail points must be integrated with the SAP system. Similarly many special function applications for unit of measure conversions (used extensively in oil & gas industry), retail point of sale systems etc. also need to be integrated into the SAP system.
All these systems have their own interfaces and sending & receiving data with a SAP system requires proper configuration in the SAP system.
SAP systems which are already in use with some business divisions of the customer will have to be integrated with the new implementation of a SAP system. Customers who have subsidiaries and different legal entities which can not be located on the same SAP system for business & legal issues must have different SAP systems and need to be integrated with the new implementation of a SAP system.
Lastly the SAP system must be able to communicate with systems implemented with customers, vendors & other partners.
Integration of various SAP & non-SAP systems is done at different levels in SAP. Many non-SAP systems have developed interfaces for integration with a SAP system. Depending on the interface available, integration may require configuration & customization of the SAP system. Integration with other SAP systems require consideration of what kind of data need to be sent & received & also which version of SAP is implemented there. Integration with customers, vendors & other partner systems can be done through EDI & other agreed protocols.