There are three methods that can be used to propagate and search for the SiteMinder User in the WebSphere User Registry based on the "AssertByUserID" setting.
By default the AssertByUserID ACO parameter for the TAI is false, and the TAI will use the SiteMinder UserDN to locate the WebSphere User in the WebSphere User Registry. If the user based on the SiteMinder UserDN will not be found in the WebSphere User Registry because the DIT structure is different, you can set AssetByUserID to "True".
With the AssertByUserID set to "True", SiteMinder will use the SMUser value to search for the user in the WebSphere User Registry. If the SMUser value won't match your User in WebSphere either, then the third option is the User Mapping feature in which you define an LDAP attribute from your SiteMinder User Directory to be used as the value to propagate and to search for the uniqueUserId from WebSphere.