It is largely the matter of the RPs.

Sun announced that it will provide the employees OpenID as long as they are employed by Sun. This is one way of doing attribute assertion.

This identifier will not be reused, Sun said.

In general, the trust model is the three way relationship amond the user-OP-RP.

By the way, there was an argument that trust can be categorized in four quadrant like this:

General
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Horizontal