Use this setting to configure the search results page used for contextual searches (such as "This Site"). This page will ...

Use this setting to configure the search results page used for contextual searches (such as "This Site").  This page will not be used for custom scopes.

By default, a Search Box will use the "Target Results Page" property in the web part. By checking the "Use site level defaults" property, the search box will use this "Site Collection Target Results Page" setting instead.