The XIF adapter presented in SAP can be used to easily communicate between SAP and non SAP systems. There are many other possibilities to communicate between SAP and non SAP systems, but in some case this technic offered by SAP can be used to quickly setup an integration solution.
This functionality can be used for inbound and outbound processing (Asynchronous). For inbound processing an IDOC, flatfile or XML message can be send to the XIF adapter. For outbound processing a XML message is created by the XIF adapter. The XIF adapter will convert the message to a BDOC in SAP CRM and than the standard process is used to create business partners, transactions etc.
I have included a full example of setting up the XIF adapter for transaction processing. This is both for inbound and outbound processing and based on the order type (additional customizing is needed for this) the order is send to the connected SAP ECC system or to the external non SAP system.
https://gstolmeijer.files.wordpress.com/2011/11/customizing-interfacing-using-xif-adapter.pdf