Dear John
only small add on regarding your topic:
"By the way, will expert rule continue to be used for future EHSM? Or maybe in the future, it can be replaced by ABAP or others? Ideas?"
1.) we can assume that SAP HANA might be used more and more and therefore the technical structure how to deal with "rule sets" might be different in the future; therefore: ma ybe in the future there wil be a different "rule engine" in place
2.) Back to the question": what is a "rule".. may be check the term: Business Rules Framework;; this is a different techniques used in other SAP modules to deal with "rules".
As mentioned. "Easy Expert" can only deal with very simple rules and very simple data constellations.
C.B.