ECO (engineering change order)
Beginner
A small fix to a nearly finished design, instead of redoing everything.
Novice
A small, targeted change to a nearly finished design, such as swapping one gate for a stronger version or adding a delay, made without rerunning the whole layout process.
Expert
Timing ECOs size, buffer or swap cells; functional ECOs patch logic; metal-only ECOs rewire spare cells placed earlier, so only metal and via masks change. Every ECO is followed by re-extraction, STA, physical verification and equivalence checking, because a fix for one check can break another.