A foreign key is a hard-constraint that must be enforced.

There's no stopping you from storing item IDs in a theoretical shopping cart table without FK constraints though. It's just that the onus is on your application to provide the guarantee that the item exists.

This becomes a little clearer if your item IDs are not simply auto-incrementing IDs (which have their own challenges in a large distributed system), but instead are SKUs, or things which have actual meaning.