SAP PI/PO is more relevant to use when your entire landscape is of SAP modules. SAP Process Integration is a part of the SAP NetWeaver platform. It is called SAP NetWeaver Exchange Infrastructure XI in NetWeaver 7.0 ehp2 and older versions. SAP NetWeaver Process Integration is a part of the NetWeaver software component and is used for the exchange of information in company’s internal system or with external parties.
SAP PI/XI enables you to set up cross-system communication and integration and allows you to connect SAP and non-SAP systems based on a different programming language like Java and SAP ABAP. It provides an open source environment that is necessary for complex system landscape for the integration of systems and for communication.
SAP Process Integration is a middleware to allow seamless integration between SAP and non-SAP application in a company or with systems outside the company. If you have a connection with cloud platforms internal or external then PI is not recommended.
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ESP like Biztalk, Fuse, Mule they are more flexible and covers larger functional areas when integration with other platforms are required. Many of these integration platforms are equipped with commercial adapters.