Once the Service Provider receives the assertion and has been configured to look for the attribute name USERID, it will grab the value and do whatever it needs to. One such real life example is SalesForce.com CRM. In OpenSSO 8 Express Build 8, there is a wizard to support easy configuration of federation with SalesForce.com which results in a map definition automatically.
One problem that i ran into (not related to the product, phew..) was that however many maps i defined i could not see them in the assertion. As a matter of fact i could not even see the