There is a workaround that we could try out. It is allowed to add a custom function as the transformation method, in case of mismatching keys. Could we prepare the function such a way that it could work for a specific doc class. @Mathias Dahl  - Any technicalcomments ?

It will not work. This is not obvious, but the job the Object Connection Transformation functionality actually does is to find, and listen closely now: what other objects to get documents, media or whatever from. So, a certain rule does not "find other documents" or "find other media". It "finds other objects". Once you understand this you will realize that a rule cannot (today) filter out certain documents or certain media, because that is now how the rules are used/work. In theory, a rule could look at that "other place" (the related object if you will) and see if it has documents of a certain kind, and decide NOT to return the ID of that object. But that would stop all documents (or media) from that object from being seen in the target.

Although a bit technical, the user documentation for Object Connection Transformation should explain this: http://docweb.corpnet.ifsworld.com/ifsdoc/Apps10/documentation/en/DefineSystemBasics/AboutObjectConnectionTrans.htm?StandAlone=true (IFS-internal URL, adjust the root part of the URL accordingly).

Now, nothing would stop us from further developing the Object Connection Transformation concept itself. To do what is asked about here, we would need to have new attributes on the rule itself, or even a child table for doing the filtering.

That would have both performance and stability problems however since the Object Connection Transformation concept is already quite heavy, performance-wise, and also complex (a lot of tricky code to get it all to work). Adding filtering on top of this would, again, be possible, but it is probably not done easily.

For these scenarios, because I think they are not that common, I would recommend using custom events to actually make copies of the document connections (or whatever) from the source to the target. In that process, the customer can decide what needs to be filtered out.