When you append a service pack, the service pack is applied to the current deployment only. Multiple service packs may be appended to a deployment. The service pack file is included in the deployment and the service pack is applied after the product is deployed.
When you merge a service pack, the service pack is merged into the administrative image. Once merged, a service pack may not be removed from the admin image. Multiple service packs may be included in single admin image.
You can choose to append or merge service packs from the Include Service Packs page when creating a deployment.