Apostle provides a nifty facility for sending app-generated, transactional emails from your application including Wordpress-like mail formatting and full tracking (delivery, opening, clicking etc.) of all emails sent. No need to set up email facilities on your app server!
This HowTo assumes that you have a smattering of ACL in general and of how the acl-extension works basically. It will introduce you into the abstract and give you a hint on how to use Business-Rules. If you have in-depth questions, please refer to the extension documentation.
Using ACL in an RBAC-manner cannot and should never obscure the fact that the underlying system is still ACL-based. In fact, the Business-Rules are just another layer in the permission-check plan.