What data is stored on a sim card?

No user data is stored in the SIM card by an iPhone. There can be contact data stored on the SIM card. iPhone can import that data. iPhone does not store user data on the SIM card. Related: https://en.wikipedia.org/wiki/SIM_card

Subscriber info for the carrier. Some phones can store contact info on the SIM, and iPhone can import that, but iPhone does not store contact data on the SIM.