The result of a call to SiteIdentityPermission.Union(System.Security.IPermission) is a permission that represents all the sites represented by both the current permission and the specified permission. Any demand that passes either permission passes their union. For example, the union of a permission that represents access to www.fourthcoffee.com and one that represents access to *.fourthcoffee.com is a permission that represents access to *.fourthcoffee.com.
The union of a permission and null is the permission that is not null. The union of a permission and a subset of that permission is the permission that contains the subset. Any other combination results in an ArgumentException being thrown. For example, the union of the site identity www.fourthcoffee.com and the site identity www.tailspintoys.com results in an exception because neither is a subset of the other.