I am a student, studying about business rules engine in different business process systems. In SAP, they have SAP Business Rules Framework Plus to act as a business rules engine.
I would like to know if BRFplus can be customized to any business process (sales, purchases...) in SAP. In general, business rules engine includes business rules, legal rules.....and it changes from one country to another. I think, it is almost impossible to develop a generalized business engine for all business processes in SAP.
BRFplus can be used in all ABAP-based business applications. To fit perfectly into the applications, BRFplus can be and has been extended by applications. A major difference of BRFplus compared with other rules engines is the built in features to extend and customize it (e.g. UI, expression and action types, exits).
To add onto what Carsten said, SAP allows people to build applications in ABAP or Java. All of SAP's most popular business processes are pre-built in ABAP, which includes it's entire ERP, CRM software etc. SAP's draw with ABAP is that your business processes are pre-built and you just need to extend them to fit your business. Java is more for the super custom business processes that cannot fit into anything pre-built.
For extending the ABAP based processes SAP provides BRFplus.
For building super custom business processes SAP also provides BRM.
So to answer your question, you can heavily extend any pre-delivered ABAP business process using BRFplus. On the Java side you can literally build anything you want with BRM.