A system identification number (SID) is broadcast by one or more base stations to identify a cellular network in a certain area (usually contiguous).
It is globally unique within AMPS, TDMA or CDMA networks (the first two systems are essentially obsolete).
The additional bit in core network protocols allows the range of codes above 32,767 to be used for internal purposes, such as segregating billing records within a large area identified by a single broadcast SID.
SIDs are assigned to every carrier (e.g., Verizon, Sprint, Alltel) by national regulators or IFAST.
Originally, in analog systems, the mobile would turn on the roaming indicator if the SID was not the single value stored in the phone.