System has various action and filter hooks throughout its files. You may browse through them and search for "hook::action" or "hook::filter" to find available hook placeholders and their parameters. Below is a list of system hooks.
system/start
System starts, no libraries or models are loaded yet.
system/controller
All default libraries and models are loaded.
system/controller/constructor
Controller is loaded but not executed yet.
system/controller/[name]
Controller's constructor is executed but not the action. [name] represents the name of the controller.
system/controller/[name]/[action]
Controller's action is executed. [name] represents the name of the controller and [action] its action (function name).
system/view/load
Controller action is executed but specified "view" file is not loaded yet.
system/view/display
View file is loaded but nothing is displayed yet.
system/end
Output is displayed.