Thank you Kirk.
I changed the IPTC/XMP preferences to "When writing IPTC/XMP - Add embedded IPTC" as well for jpegs, tiff as for tiff-based RAW photos. I left the "if XMP sidecar exists, add and/or update with IPTC4XMP" setting like it was. So this way, only my IPTC data will be stored with the nef or the jpg, alterations by ACR in a sidecar? Correct so far?
Using tool > delete metadata > xmp (deletes embedded and sidecar file data) will delete all the xmp sidecars from my pictures over the past year since I've been using ACR. Is there any way of just deleting the embedded data so ACR would recognize the original xmp sidecar again?
Regards,
Oliver