Some orders are treated as final even though the case continues, because deferring review would make it meaningless.
Three requirements. The order must conclusively determine the disputed question; resolve an important issue completely separate from the merits; and be effectively unreviewable on appeal from a final judgment.
Recognised categories. Denials of qualified immunity on legal grounds; denials of sovereign or Eleventh Amendment immunity; orders on double jeopardy; and certain orders regarding the right to a public trial. The third requirement is what unites them — an immunity from suit is destroyed by having to stand trial, so post-judgment review cannot restore it.
Categories rejected. Discovery orders, including those said to compel privileged material, are generally not immediately appealable; the route is mandamus or contempt. Disqualification of counsel, class certification, and denial of summary judgment on the merits are likewise not covered.
Fact-bound immunity denials. A denial of qualified immunity resting on the existence of disputed facts is not immediately appealable; only the legal question of whether the assumed facts violate clearly established law is.
Strategic caution. An unsuccessful collateral order appeal costs time and may draw an award of fees for a frivolous appeal, and it does not stay the trial court proceedings automatically.