Hi Gopi,
You cannot directly reference a named expression in a SAML partnership. Instead, modify the user directory which your partnership references. In the user directory, scroll to the Attribute Mapping List section at the bottom and create a user attribute mapping of type Expression. Within the Definition field, you can use expressions, or simply enter the named expression you would like to reference. When finished, save your change to the user directory.
Previously I erroneously called this attribute mapping a virtual user attribute. The named expression itself is the virtual user attribute, and the attribute mapping is defined in the user directory object. Only through an attribute mapping can you make use of the standard expression language and named expressions.
Now that the attribute mapping is defined as part of your user directory, you can use it in your SAML partnerships just like any other user attribute. The attribute type will be User Attribute, not Expression. For the Value of your assertion attribute, use the name of your attribute mapping, not the name of your named expression. For example, if I were to build a SAML partnership using the attribute mapping above, the Value field of my assertion attribute would be myAttribute.
Within the policy store, a SAML partnership uses various different objects including a backing affiliate domain, which is not an application. The Admin UI obfuscates these inner workings, but they can be seen in policy store XML and in XPS tools. Regardless, you can do all of the above within the Admin UI to customize the content of your assertion attributes or name ID.
I hope this helps clarify.
Daniel