I'm not surprised you find it unclear; it seems pretty unclear even to many professionals. "NOS" has been a catch bin diagnosis. It can represent elements of a personality disorder are present but not enough of one particular disorder to diagnosis it (but several elements from several different disorders). Or it may indicate more than one disorder is present, a mix.
The profession in general is looking at revising how personality disorders are diagnosed, as nobody seems terribly satisfied with the current way its done. This article, Research led by Michigan State University psychologist Christopher Hopwood proposes a new way to classify personality disorders
, might be informative.