In practice
A publicity still is the public-facing end of the stills process. It begins life as one frame among thousands from unit stills photography, survives the edit to become a select, and is published only once it has cleared approval.
That approval is what separates a publicity still from any other shot. Through talent approval the cast sign off the frames they appear in, and cast clearance confirms the image may be used for publicity. A frame an actor has killed never becomes a publicity still.
In use, publicity stills appear in posters, press packs, streaming artwork, listings and social campaigns. They are delivered as the approved key selects: high-resolution, correctly captioned and credited files ready for distributors and press offices to drop straight into a campaign.
In short, every publicity still is an approved still — chosen for impact and cleared for release. For how they are prepared and handed over, see our guide to delivering approved stills.