Poking around in the class hierarchies, package infos, categories etc. 
I found that the SystemOrganizer has this interesting comment 
(SystemOrganization commentRemoteStr):

  'These categories are only intended as a guide.  It is OK to put a 
class in several categories, and this will allow you to maintain a 
category (e.g. Current) for the classes with which you work most 
frequently.  Observe the paren and quote form when editing system 
organization.'

I don't know the history of this comment, but SystemOrganizer surely 
does not work like this anymore. Either we fix the system or the 
comment. I'm leaning towards the latter ...

- Bert -