Hi,
The following text is come from Dev guide - security and permissions, but I don't really understand, so please help me to explain it.
"This mechanism allows a common capability-style model where user interaction (opening an attachment, selecting a contact from a list, etc) drives ad-hoc granting of fine-grained permission. This can be a key facility for reducing the permissions needed by applications to only those directly related to their behavior."

In the context of the given example in that text.. its simple.
The particular part you want to know just points out the pros
of the per-URIpermission. If you use it right your app needs less
permissions above all none that could seem suspicious to the user.
In the given example of the text your image viewer app does not need
a permission to read all email. Users would think its weird i guess if your image
viewer app would need this permission. To show that image without that permission
your app gets a URI permission on startup that allows your app to access that particular image
as some sort of exemption. Its a way of borrowing permissions from one app to
another.. so to say.

I hope this clears it up