If I have few plugins in 2 different categories and want to assign them also to the new one after selection I have <multiple categories> what sounds logical but after adding new one they are removed from their original categories and assigned to real created category named <multiple categories>
 
For instance before:
Guitar amps
   Plugin A
Guitar cabs
   Plugin B
 
After selection and adding new category "Set A" I got in plugin browser:
<multiple categories>
   Plugin A
   Plugin B
Set A
   Plugin A
   Plugin B
  
So in fact I lose info about their original assignment and must set prime category as 'Set A', remove category  '<multiple categories>' and add them again to their original. What lucky I am I did it only on 4 plugins not for instance on 40 :)