the main thing is the scope
-- objects stored in the application scope can be accessed by any page/any user in the web app.
-- objects stored in the pageContext scope can only be accessed by that user from that particular page translation
unit.
(and remember... a page translation unit it aany individual page that is translated... so a statically included page will be considered part of the parent page's Page Translation unit, but a dynamically included page will be a seperate Page Translation Unit.)