qmk_firmware/data/mappings/defaults.json

{
    "development_board": {
        "promicro": {
            "processor": "atmega32u4",
            "bootloader": "caterina",
            "pin_compatible": "promicro"
        },
        "elite_c": {
            "processor": "atmega32u4",
            "bootloader": "atmel-dfu",
            "pin_compatible": "promicro"
        },
        "proton_c": {
            "processor": "STM32F303",
            "bootloader": "stm32-dfu",
            "board": "QMK_PROTON_C",
            "pin_compatible": "promicro"
        }
    }
}