From the Mainmenu I when selecting Categories I know this runs category.php. How are the routines and libraries from K2 getting loaded prior to category.php running? I want to know this because variables within category.php are already available, but I don't see how or what gets loaded before this program is called.

Can someone please explain this? What files are loaded and where are they located for K2. I need to know this because not everything is loaded it appears such as in administration K2 pages it doesn't have the same variables as category.php for accessing K2 Items and K2 Categories.