The file Poser.RSR is indeed crucial to Poser. It just won't run without it! Actually I've write protected mine (and I would suggest you to do the same) so that it is kept safe. Afaik the POSER.RSR is a third kind of RSR, different from the geometry rsr's and the thumbnail rsr's.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.