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).
(On older devices, especially feature phones, it also stored things like text messages and contacts). The SIM card allows you to change cellular phones and keep the same user data. The SIM card is what lets your carrier know that you're the account holder of the iPhone using it.So if you take your SIM card our of your iPhone and then put it into your friend's iPhone and receive calls as though it were your own (assuming the SIM card is compatible).On older devices, especially feature phones, it also stored things like text messages and contacts). The SIM card allows you to change cellular phones and keep the same user data. The SIM card is what lets your carrier know that you're the account holder of the iPhone using it.