A general rule of thumb is that if your economic model has no value judgements, it's positive economics, and if it does have value judgements it's normative economics, since to tell someone what he ought to do, you first have to judge what is best for him.
A subscriber identity module or subscriber identification module (SIM) is an integrated circuit that is intended to securely store the international mobile subscriber identity (IMSI) number and its related key, which are used to identify and authenticate subscribers on mobile telephony devices (such as mobile phones and computers).
An iPhone SIM card is a standard size card that looks similar to an SD card. When you subscribe to any GSM carrier, you will be provided with a SIM card that holds your cell phone information such as your phone number and can even store your contacts. In the US an iPhone data plan is required with any SIM card you get for the iPhone.