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 last digit of CID represents cells' Sector ID:[1]

    value 0 is used for omnidirectional antenna.
    values 1,2,3 are used to identify sectors of trisector or bisector antennas.

source: wikipedia