Simply put, the Unseen were a bunch of 'Mechs (although the list is smaller now...there were also at least one fighter and maybe a couple other things that could have been involved) whose images were not created by FASA, but taken from other sources, primarily shows that were Japanese in origin...legal action took place between FASA and another American company that felt it owned sole rights to all images from said shows...eventually the images (and even the mention) of these 'Mechs was removed from BTech because of this, as well as those images of any other 'Mechs that FASA did not create in house and solely own...
The Reseen refers to new models of these 'Mechs, etc. which have had new artwork made for them that pays homage to the original artwork, but is sufficiently different as to no pose legal issues...
We still have rules against showing images of the Unseen in posts made here so that there is no (or at least limited) possibilities of legal action taking place...
Ruger