A scannable QR code that carries your full tagged campaign URL.
A QR code is just a scannable version of a URL — but if that URL carries UTM parameters, every scan shows up in your analytics tagged to the exact source. That's what turns a flyer, storefront sign, or print ad from an untrackable guess into a measurable channel.
Without tagging, a QR scan just looks like generic direct traffic in GA4 — you'll never know it came from the poster outside your store.