A mobile advertising identifier (MAID): Apple's IDFA (`type` `idfa`) or Google's advertising id (`type` `adid`), as a lower-case UUID. `00000000-0000-0000-0000-000000000000` means the user opted out of tracking; every opted-out device shares it, so it identifies no device. `00000000-0000-0000-0000-000000000000` is a valid value and can appear in outputs.