There is a high likelihood that many PDS systems will provide other services to the user, such as a photo hosting service, blog, social network, E-mail, etc. The service provider can list those services, and ask the user to to confirm they are in fact okay with sharing that information.
However, a much more interesting feature is for the PDS to allow the user to specify what other websites they use or are subscribed to, such as their favorite social network. The flow might work as follows:
Sara is a gmail.com user and has been using gmail.com as her PDS
She logs into Facebook which detects that gmail.com is her PDS, and requests her public attributes, and sees that Facebook is not listed as one of the websites she uses
Facebook then shows a promotion on their site which ask the user "Would you like other websites to know you are a Facebook member so that you can interact with your friends on those sites?"
If the user clicks that option, then they are redirected to her PDS in a standardized manner which requests that Facebook be added to her public attributes. Gmail.com would then show a confirmation page asking "Please confirm that you would like all the websites you visit to know that you use www.facebook.com"
Sara clicks yes, and that information is now added to her PDS preferences
The next day she visits gothamtimes.com and is reading an article. She notices on the side the option to "Share this article with your Facebook friends" so she clicks it, and gothamtimes.com now uses Facebook's proprietary API to get the user's consent to have a message relayed by Facebook to her friends