In APEX 4 you can define Authorization Schemes. This is a very useful feature to prevent code repetition all over the place. For instance and Authorization Scheme "Is Admin" might use a select on one or more tables (or web service calls or whatever is necessary) to determine whether a user, the :APP_USER, has an Admin role or not. And you can use any value in session state, like :APP_ID or :APP_PAGE_ID in your query (or function call). The result of this call is usually pretty static. So you could specify when the code should be evaluated: Once per Page View or Once per Session. The latter is obviously more efficient as it will run only once from login to logout.
This works fine. Until you want to build your own fine-grained access control mechanism. As an example: If you have a page with three buttons on it, you can define an Authorization Scheme for this buttons and use that. So all three buttons use the same Authorization Scheme and are all visible on the page or not. As the Authorization Scheme will be evaluated once and only once for that Page. And if you need more fine grained controle you had to define three different Authorization Schemes, one for each button. And that will grow into a maintenance nightmare.
In APEX 5 this will be resolved. Next to the "old" options you can now specify an evaluation "Once per Component" and "Always".
"Once per Component" means the code is evaluated once per component for the duration of the session. So using this setting you can (re)use the same Authorization Scheme for the three buttons - as it will be evaluated three times. To make it even more useful : They also included three new bind variables (:APP_COMPONENT_TYPE, :APP_COMPONENT_ID and :APP_COMPONENT_NAME) that you can use in your query. So using these new bind vars (or one of them) you can create functionality that supports things like DYI fine grained access control - where a privileged user can grant or revoke access to certain elements on a page!