Glossary
Assurance Level
An assurance level describes how much confidence a particular authentication method provides that the party authenticating is genuinely who they claim to be — a single factor typed from memory sits at one tier, and a hardware-backed key pair combined with a second factor sits at a considerably higher one. Treating every successful login as equally trustworthy, regardless of the method behind it, ignores a real and often consequential difference.
Matching required assurance level to the stakes of what's being authorized is the practical use of the concept: a low-stakes action can reasonably accept a lower tier, while an action with serious consequences warrants requiring a higher one, even if that means the same user has to prove themselves more rigorously for that specific action than they did just to log in.