Suppose I tag a Group with “abc”, now I do not want the documents I put in this group to have tag 'abc" is that possible ? In other words do the children always inherit the parent’s attributes ?
Thanks
Documents inherit the tags of the groups they are in – documents in child, grandchild groups, etc, inherit also the tags of their parents, if any. When the document is removed from the group, the inherited tags are removed. If groups are enabled for tagging, then the group-tag is also inherited by children, grandchildren, etc., and also removed when the document is moved away from that group hierarchy.
These things are always easy to test for oneself of course. 
Yes thats right so there is no way to decide whether or not you want the inheritance to take place or not …I had thought that may there were a function to allow me to decide …NOt a big deal anyway just a question of structuring things properly, not so easy …
Just a note: If you think about your question, the actual behavior is a purely logical thing. When you apply a Tag to a Group, you are telling it, “I don’t want to individually Tag all the items in here, so just Tag them all for me.” 