The $Resource replacement system is populated from the Language files loaded to the CRM Global Manager control. Once USD determins the language of the user, it attempts to load the language resource for that language in the $Resource object.
If you need to localize text or other objects you can use the $Resource replacement param for display or uri.
for example
using en-us , the [[$Resource.SEARCH]] == "Search"
where as with fr-fr, the [[$Resource.SEARCH]] == "Rechercher"
to add resources, you just need to add them to the language resource web resources stored in CRM
Mattb.