Artwork is the full-bleed image behind an Apple Wallet pass rendered in the poster layout. It fills the pass edge to edge, with the fields laid over it, and it is what makes a poster pass a poster rather than a banded card with a picture at the top. Where the strip image is a banner inside a fixed layout, artwork is the layout.
Artwork ships inside the PKPass bundle and is declared in pass.json alongside the classic layout, so a device that does not render poster passes falls back to the banded design and its strip image. That means a poster pass carries both sets of imagery, and both have to be art-directed, not just the one you designed first.
The demands are higher than for a strip. The image has to work at pass proportions, hold text over its whole surface rather than in one band, and stay legible on the lock screen and in a wallet stack where it is seen small and in passing. Contrast has to be controlled across the entire frame, and the field colors chosen against the image rather than against a brand palette in isolation.
Artwork is also where a loyalty or membership program can differentiate. Passes have looked broadly alike for a decade; a full-bleed surface, especially one refreshed for a season or a campaign, is the first real chance to make a pass feel designed rather than issued.