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).
SIM cards are the heart of a cellular phone, containing the phoneâs number, carrier designation, rate plan, address book, security codes, and more. SIM cards are only used in GSM-enabled phones. SIM cards can be moved from phone to phone within the same carrier. Some users may opt to purchase a prepaid SIM card to avoid the necessity of signing a long-term contract with a carrier.