I have a two questions about custom search scopes. A user wants to have a custom
scope that start search from their top site and sub sites. I have created the
scope and using the scope rule as web Address in which the URL of their top
site. User wants this scope to be the default scope on the search drop down. My
questions are:

1. What is the difference between shared scope and non-shared scope? Do I need
to make this shared? if so Why?

2. Is it possible to make this custom scope, be the default scope for their site
and sub sites? In other words, in the search drop down, the default selection
should be this custom scope instead of "All Sites". Important: It should not
appear on the root of the site collection. This site is not a separate site
collection? If it's possible how to do that? I have seen online instructions on
updating the SearchArea.xml file to set the current site as default scope and
that's not what the user wants here.