WordPress tries to be event driven with all it's actions and filters you can hook to. But it also has special functions to create admin pages such as add_submenu_page() which accepts a (callback) $function param.
When does this function actually execute? Between or at which action trigger time?
It must be pretty late in the action order because I've tried hooking to admin_notices from it and it doesn't work. See Actions Run During an Admin Page Request for the typical action triggering order when in /wp-admin .
Why do I want to know this? I'm processing a form from the callback of add_submenu_page() because it's the best place I could think of to process the form POST data, and on validation errors I want to display a simple admin notice "the right way" which is by hooking a fn into admin_notices. I guess I could just print the notice in a conditional within my html code, or try to process the form at an earlier stage.
Note: I don't like the idea of using admin_post_(action) instead of the callback of add_submenu_page(), it's not useful.