Hi Toomas,
I am really no PSU expert, but IDI certainly stores all XMP to the database and one can also configure it to show any XMP field.
I assume that is the same in PSU. How else would you explain that if you have not imported the XMP to PSU after changing the files in LR4 they revert back to their original setting? That can of course only happen because the previous settings were stored from XMP to the database and those previous settings are then written back to the file, thus overwriting the "newer" LR4 settings.
IDI or PSU of course cannot "understand" the LR settings, because it is a different kind of software and algorithms that LR4 uses to adjust images are proprietary. So, you can configure IDI to make this this xmp visible (it is always already there under under the advanced metadata branch), but will most likely not understand it. It will look similar to the "gibberish" that you see when you look at IDI's proprietary ICS XMP schema.
Still, that these settings can be "seen" by IDI is useful, because IDI then "knows" the adjustments are there and updating thumbs will reflect the edits to LR provided that LR edits also resulted in the embedded thumb getting updated.
How PSU does that I do not know. I assume it will be similar.
Frank