A GSM Cell ID (CID) is a generally unique number used to identify each base transceiver station (BTS) or sector of a BTS within a location area code (LAC) if not within a GSM network.

In some cases the first or last digit of CID represents cells’ Sector ID:

value 0 is used for omnidirectional antenna,
values 1, 2, 3 are used to identify sectors of trisector or bisector antennas.
In UMTS, there is a distinction between Cell ID (CID) and UTRAN Cell ID (also called LCID). The UTRAN Cell ID (LCID) is a concatenation of the RNC-ID (12 bits, ID of the Radio Network Controller) and Cell ID (16 bits, unique ID of the Cell). CID is just the Cell ID. The concatenation of both will still be unique but can be confusing in some cellid databases as some store the CID and other store LCID. It makes sense to record them separately as the RNC ID is the same for many cells, the unique element is the CID.

A valid CID ranges from 0 to 65535 (2¹⁶ − 1) on GSM and CDMA networks and from 0 to 268435455 (2²⁸ − 1) on UMTS and LTE networks.

Source: Cell ID – Wikipedia