They are not required. It's an optional thing that I do when I create the default values so that I can visibly see when a default value is loaded. This was primarily when doing development of the extension so I could tell the difference in the designer when
default values where used which effectively means there's a problem in the provider or the key.
In general I think this is a good idea for development as well though. If you end up rendering a default value it means that something is wrong with the binding/provider and it needs to be fixed. If the default value and the default local culture value are
the same you never know there's a problem until you switch into another language... The marker character lets you see immediately that there's a problem that needs to be addressed while still providing a usable placeholder with a realistic value.
But - totally optional. You don't have to use any special characters or you can choose what characters you use.
Hope this helps,
+++ Rick ---