+There are several functions that you can define in your keymap to customize the functionality of this feature.
+
+* `void qk_ucis_start_user(void)` – This runs when you call the "start" function, and can be used to provide feedback. By default, it types out a keyboard emoji.
+* `void qk_ucis_success(uint8_t symbol_index)` – This runs when the input has matched something and has completed. By default, it doesn't do anything.
+* `void qk_ucis_symbol_fallback (void)` – This runs when the input doesn't match anything. By default, it falls back to trying that input as a Unicode code.
+
+You can find the default implementations of these functions in [`process_ucis.c`](https://github.com/qmk/qmk_firmware/blob/master/quantum/process_keycode/process_ucis.c).
+
+## Input Modes
+
+Unicode input in QMK works by inputting a sequence of characters to the OS, sort of like a macro. Unfortunately, the way this is done differs for each platform. Specifically, each platform requires a different combination of keys to trigger Unicode input. Therefore, a corresponding input mode has to be set in QMK.
+
+The following input modes are available:
+
+* **`UC_OSX`**: macOS built-in Unicode hex input. Supports code points up to `0xFFFF` (`0x10FFFF` with `UNICODEMAP`).
+
+ To enable, go to _System Preferences > Keyboard > Input Sources_, add _Unicode Hex Input_ to the list (it's under _Other_), then activate it from the input dropdown in the Menu Bar.
+ By default, this mode uses the left Option key (`KC_LALT`) for Unicode input, but this can be changed by defining [`UNICODE_KEY_OSX`](#input-key-configuration) with another keycode.
+
+ !> Using the _Unicode Hex Input_ input source may disable some Option based shortcuts, such as Option + Left Arrow and Option + Right Arrow.
+
+* **`UC_LNX`**: Linux built-in IBus Unicode input. Supports code points up to `0x10FFFF` (all possible code points).
+
+ Enabled by default and works almost anywhere on IBus-enabled distros. Without IBus, this mode works under GTK apps, but rarely anywhere else.
+ By default, this mode uses Ctrl+Shift+U (`LCTL(LSFT(KC_U))`) to start Unicode input, but this can be changed by defining [`UNICODE_KEY_LNX`](#input-key-configuration) with another keycode. This might be required for IBus versions ≥1.5.15, where Ctrl+Shift+U behavior is consolidated into Ctrl+Shift+E.
+
+* **`UC_WIN`**: _(not recommended)_ Windows built-in hex numpad Unicode input. Supports code points up to `0xFFFF`.
+
+ To enable, create a registry key under `HKEY_CURRENT_USER\Control Panel\Input Method\EnableHexNumpad` of type `REG_SZ` called `EnableHexNumpad` and set its value to `1`. This can be done from the Command Prompt by running `reg add "HKCU\Control Panel\Input Method" -v EnableHexNumpad -t REG_SZ -d 1` with administrator privileges. Reboot afterwards.
+ This mode is not recommended because of reliability and compatibility issues; use the `UC_WINC` mode instead.
+
+* **`UC_BSD`**: _(non implemented)_ Unicode input under BSD. Not implemented at this time. If you're a BSD user and want to help add support for it, please [open an issue on GitHub](https://github.com/qmk/qmk_firmware/issues).
+
+* **`UC_WINC`**: Windows Unicode input using [WinCompose](https://github.com/samhocevar/wincompose). As of v0.9.0, supports code points up to `0x10FFFF` (all possible code points).
+
+ To enable, install the [latest release](https://github.com/samhocevar/wincompose/releases/latest). Once installed, WinCompose will automatically run on startup. Works reliably under all version of Windows supported by the app.
+ By default, this mode uses right Alt (`KC_RALT`) as the Compose key, but this can be changed in the WinCompose settings and by defining [`UNICODE_KEY_WINC`](#input-key-configuration) with another keycode.
+
+### Switching Input Modes
+
+There are two ways to set the input mode for Unicode: by keycode or by function. Keep in mind that both methods write to persistent storage (EEPROM), and are loaded each time the keyboard starts. So once you've set it the first time, you don't need to set it again unless you want to change it, or you've reset the EEPROM settings.