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.
SHA-256 of a normalized email, `SHA2(NORMALIZE_EMAIL(email), 256)`: lowercase hex, 64 characters. Hashes of un-normalized emails won't join. The SHA-256 of an empty string, `e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855`, isn't a hash of any email and reads as NULL.
An email address hashed with one of several message-digest algorithms; the specific algorithm is carried in the type property of each value. Prefer the algorithm-specific attributes when the algorithm is known: md5_hashed_email (74), sha1_hashed_email (75), sha256_hashed_email (76). Hashed emails only join when both the algorithm and the input normalization match.