Quantum keycodes allow for easier customization of your keymap than the basic ones provide, without having to define custom actions.
All keycodes within quantum are numbers between
0xFFFF. Within your
keymap.c it may look like you have functions and other special cases, but ultimately the C preprocessor will translate those into a single 4 byte integer. QMK has reserved
0x00FF for standard keycodes. These are keycodes such as
KC_LCTL, which are basic keys defined in the USB HID specification.
On this page we have documented keycodes between
0xFFFF which are used to implement advanced quantum features. If you define your own custom keycodes they will be put into this range as well.