I'll respond to your comment in another answer as the comment doesn't give sufficient space.
The layout handle defines which pages a block will be added to, so if you look further up
page.xml above the section you have posted, you see a
<default> tag. This is opening layout handle tag and using default means apply the layout changes here to every page.
If you look at other layout files you will see other handles for instance in
catalog.xml you find
<catalog_product_view> which is the layout handle for product view pages and any layout items defined inside this handle will be only be used when viewing a product. The layout handle is created from an underscore separated concatenation of module name, controller name and action name, so with the above, the module is catalog, controller is product and action within that controller is view.
This gives you the scope you need to make layout changes just on particular pages from your own layout files. It's worth noting also that if you are not sure how to determine the layout handle for a particular page, it's inserted as the first class by default into the
<body> HTML tag on each page.
Note that frontend and admin pages have separate layout files so the
<default> tag only applies to either the frontend, or admin dependant on which layout file it has been placed in.
I would highly recommend you create your own
local.xml layout file (or if you are creating an extension define your own layout file in it's
local.xml is much cleaner way to manage layouts - you leave default layout files untouched and manage all layout changes, using handles, through a single file. If local.xml doesn't exist in your themes layout directory just create it and it will be automatically picked up. Here is a
local.xml layout file with some example syntax to get you started:
<block type="page/html" name="new.block" after="-" template="module/block/template.phtml" />
<remove name="some.block.name" />
<block type="page/html" name="other.new.block" template="module/block/other_template.phtml" before="block_name_to_appear_before" parent="content" />