+When undefined:
+
+- `USR1` defaults to `KC_SPC`, otherwise use `CFQ_USER_KEY1`.
+- `USR2` defaults to `KC_ENT`, otherwise use `CFQ_USER_KEY2`.
+- `USR3` defaults to `KC_FN3`, otherwise use `CFQ_USER_KEY3`.
+- `USR4` defaults to `APP`, otherwise use `CFQ_USER_KEY4`.
+