This field is used to link the product to another business object. it is very convenient to navigate to target business object via this kind of extension field.
In AET, Choose Application Reference as field type and opportunity as sub type.
Once you create new product, you can manually assign an opportunity to this extension field via F4 value help:
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SAP CRM AET Application Reference类型扩展字段的一个例子
原文：https://www.cnblogs.com/sap-jerry/p/12232824.html