Yes Kate, that's it exactly.
It's not so much a 'parameter' like a post count, more an 'editorial' decision on the part of the Forum moderator or Webmaster that when they see an interesting Topic developing, which takes on a life of it's own, they could decide the subject matter is important enough to become a Forum in it's own right.
Therefore, the new Forum could be created and old Topic could be 'moved' to become the first in the new Forum. It's the proceedure of moving a Topic 'automatically' which I would like to achieve, without 'manually' transferring every message by hand (even if that were possible).
Hope that makes sense!