dev/tools/gen_keycodes/data/mask_constants.json

{
  "valueMask": {
    "value": "0x000FFFFFFFF",
    "description": [
      "Mask for the 32-bit value portion of the key code.",
      "This is used by platform-specific code to generate Flutter key codes."
    ]
  },

  "platformMask": {
    "value": "0x0FF00000000",
    "description": [
      "Mask for the platform prefix portion of the key code.",
      "This is used by platform-specific code to generate Flutter key codes."
    ]
  },

  "unicodePlane": {
    "value": "0x00000000000",
    "description": [
      "The code prefix for keys which have a Unicode representation.",
      "This is used by platform-specific code to generate Flutter key codes."
    ]
  },

  "autogeneratedMask": {
    "value": "0x10000000000",
    "description": [
      "Mask for the auto-generated bit portion of the key code.",
      "This is used by platform-specific code to generate new Flutter key codes for keys which are not recognized."
    ]
  },

  "synonymMask": {
    "value": "0x20000000000",
    "description": [
      "Mask for the synonym pseudo-keys generated for keys which appear in more than one place on the keyboard.",
      "IDs in this range are used to represent keys which appear in multiple places on the keyboard, such as the SHIFT, ALT, CTRL, and numeric keypad keys. These key codes will never be generated by the key event system, but may be used in key maps to represent the union of all the keys of each type in order to match them.",
      "To look up the synonyms that are defined, look in the [synonyms] map."
    ]
  },

  "hidPlane": {
    "value": "0x00100000000",
    "description": [
      "The code prefix for keys which do not have a Unicode representation.",
      "This is used by platform-specific code to generate Flutter key codes using HID Usage codes."
    ]
  }
}