Hi Denes,
I don't know where is the official doc, but quick googling gives these links:
http://comments.gmane.org/gmane.comp.emulators.vmware.tools.user/121
Notice that first link talks about persistent vs. non-persistent properties. I didn't try it myself, but the description makes sense.
From within customization we use something like:
${CMD_VMWARE_GUESTD} --cmd "info-set guestinfo.gc.status '$message'"
that's how we can report an error.
/Andrii