When this.root equals null or not!

Hello

I have some confusions on root and the stage classes; like;[LIST=1]
[]When should I use root or the stage class, from within my custom classes, in order to access the existing objects on the main timeline.
[
]When is this.root == null or not!
[*]How can I check numChildren on the main timeline.
[/LIST]I will highly appreciate any help & guidance on the subject, as a whole or on the mentioned points in particular.