Lines like key [ Cyrillic_ie, Cyrillic_IE, Cyrillic_io, Cyrillic_IO]; are not incomprehensible as well. This particular line instructs that a press on physical key will normally emit lower-case 'Cyrillic_ie' character. Upper-case character 'Cyrillic_IE' will be emitted when will be pressed while physical key will be held down. Similarly, a press on physical key will emit 'Cyrillic_io' or 'Cyrillic_IO' depending on the key state provided that an extra modifier key will be held down. Such a modifier key is addressed as 3rd level chooser. You can select this extra modifier key according to your taste later in System > Preferences > Hardware > Keyboard > Options dialogue.