Dear Christoph
Edward explained the most important topics regarding use of REAL_SUB_E. From high lkevel point of view: using REAL_SUB_E one you can "maintain" any data which you get from supplier and attach any document on it (like MSDS/SDS etc.). If you use e.g. a material type like "RAW" the approach to use REAL_SUb might be a good one. You have one EHS object, you have one MM object and you can "control" better data maintenance as well as processes. Using the REAL_SUB you can create "links" to the REAL_SUB_E (by using e.g. composition etc.).
Alternartive: if you have one or more supplier you could enter the data as wellon level of REAL_SUB using either no "usage" or based on your concept of using the usage you could use a combination of rating / validity are which does have no effect on other subsequent processes. The Document can be stored as well on level of REAL_SUB and by usngf the existing document class you can "highlight" which document has been received from which supplier. If you use SAP Producta and REACh Copmplaince suite may be you should pay attention. I am not 100% if you can avoid the use of REAL_SUB_E.
I "googled" a little bit anf dound on the topi these documents:
REACH : Creation of REAL_SUB_E & Linking wi... | SCN
consolut - Specification Types (Changed) ( RELNEHS_606_EHSCI3_SPECN )
consolut - Technische Harmonisierung (Erweitert) ( RELNRCS_TECH_HARM )(sorry: only german version)
C.B.