When configuring Security Filtering settings on a GPO, what you are really doing is changing security permissions on the GPO itself. You see, for a computer or user to be able to apply any given Group Policy Object, that computer or user needs to have two different "rights" to the GPO. It needs to be able to "read" the GPO, and it needs to be able to "apply group policy".
You can view the permissions for any GPO by clicking on the GPO (or any one of its links), then visiting the Delegation tab. Here, you can see a general overview of the permissions on the GPO, but you won't see the full story until you then click on the Advanced... button. Clicking Advanced... shows you the full picture of permissions ...
