If your service requires only a small amount of information from an OpenID provider (such as a thumbnail, profile link, name, and some other basic public badge information), Simple Registration is a wonderful extension that delivers that information to the application after the user has logged in to the provider site.
The Simple Registration extension defines nine common user profile fields that a provider may be requested to supply through the OpenID authentication process. Note, however, that the Simple Registration specification does not require a provider to support all nine fields; the provider may support any number of them.
Many providers opt to support the Attribute Exchange extension in lieu of Simple Registration because Attribute Exchange defines not only the fields supported by the Simple Registration extension, but also a large number of others. This does not mean that no OpenID providers support Simple Registration; it simply means that some opt to support only the more comprehensive extension instead of both.
To this end, Table 11-3 shows the nine fields that Simple Registration supports and then, for comparison, ties in the matching supported field URIs for the Attribute Exchange extension.