Microsoft Knowledge Base Article 324750 contains the following summary:
You (as an administrator) can use group policies to assign or publish software to users or computers in a domain, and it is useful to be able to deploy software based on group membership. Group Policy Objects (GPOs) are typically applied only to members of organizational units to which the GPO is linked. Because users cannot be located in several organization units at one time, you must be able to apply group policies outside the boundaries of organization units. This article describes how to apply your software deployment policy to users who are outside the boundaries of organization units.