Manage your WordPress Widget Areas and additional settings related to page templates and widgets.

This allow you to create widget “zone” or “group” to use later on in different section of your site (Appearance > Widgets)

The drop downs such as Single Post Default Sidebar, Single Video Default Sidebar, Single Event Default Sidebar, etc, apply ONLY to NEW post. When you create a new post (or event or video or photo album, etc..) this will set a default sidebar automatically instead of selecting which sidebar you want for your different post templates.