Scope has nothing to do with what page a portlet is on.
Web content portlet can display different content anywhere in the portal. Has nothing to do with scope, it has to do with configuring each instance to choose the content to display. This information is stored in the portlet preferences (these are per-instance preferences so they only apply to the individual instance and do not affect other instances).
You can find out the meaning for all the tags by referring to the DTDs and XSDs; it's the one place where Liferay is really good about documenting APIs.
