edit >> delete by type>> history

basically when you build something it keeps a record of what you did. You can then go back and edit parametres and adjust things. I have never found it to be all that useful though (I think max's stack is better).

You need to be careful when you do it though as it can cause problems (e.g. if you delete the history of an instanced object it will lose all of the instance properties so any changes made to the original wont affect it)

That's a basic summary of it

Alan