Understading meaning of KEY

Dear root expert,
I would like to ask you if you could clarify the meaning of the KEY word. For example, consider
KEY: TTree tree;4 tree
KEY: TTree tree;3 tree

  1. Which is the difference between tree;3 and tree;4.
  2. Is it true that there could be a difference in the number of events?
  3. Why there are 2 KEYs and which one is correct to use?
  4. If I want to call such a tree is it correct to use only
    instead of
    f->GetObject(“tree;3”,tree); or f->GetObject(“tree;4”,tree); ?
  5. Are the same differences valid also for directory? E.G.
    KEY: TDirectoryFile Selection;1
    KEY: TDirectoryFile Selection;2

Thank you very much for helping.

[url]Aux;1 aux;2 aux3;

Thank you very much.