A placeholder address is what a person types into an address field when they do not intend to give one: "asdf", "test", "n/a", "no address", "same as above", "123 Fake Street", "Anytown" — a word chosen to satisfy the form rather than to name a place, and one that no register, directory or operator list will ever confirm.
What Spaw answers
looks_placeholder is true when the street or the town of a valid address is a whole placeholder name or a shape nobody would give a place — four or more letters with no vowel, one character repeated, digits alone, a run along a keyboard row — false when neither is, and null when there is no street and no town to judge. A true answer carries the placeholder signal, weight 60, citing the address-placeholders dataset in sources, which is enough for risk_level: high on its own, and the default shipping policy blocks it.
What it does not mean
Only whole names count. "Test Valley Road" is a real road in Hampshire and "Sample Road" exists in several American towns; each folds to a different key from the words "test" and "sample" alone, so neither is flagged, and a short real name like Rye or Ely is never judged. A placeholder is also not a typo: a misspelt real street answers street_suggestion from the register, not this flag.