Apple’s cloud services (iTunes, iCloud, etc.) require you to create an Apple ID, which is essentially your key to access. Each person using Apple’s services should have a unique Apple ID, but sometimes people share IDs for convenience. That could led to some issues, however.
For example, if an Apple ID is being shared by a couple, it can create a problem if the couple breaks up. A reader writes in with such a story:
My partner and I broke up. We’ve been sharing the same iTunes account for years. Everything downloaded on one phone appears on the other. How do I stop this?
While the question is about iTunes, an iTunes account is a form of Apple ID: you can use it just for purchasing, or with iCloud for calendar, contacts, email, photos, and music, as well as macOS access and elements associated with Continuity (iOS/macOS interactions).