A small number of proper shipping names in the Hazardous Materials Table are assigned identification numbers preceded by the letters 'ID' instead of UN or NA. What does the ID prefix indicate?

49 CFR 172.101(e) — identification numbers column

You won't see ID-prefixed numbers as often as UN or NA, but recognizing what the prefix signals keeps you from assuming it's a typo or an unclassified material when you do encounter one on a shipping paper.