NCC is part of BSIC. MS uses it to distinguish adjacent BTS that belong to different GSM PLMN. NCC is composed of 3 bits, with coding capacity of 8.
NCC and BCC together constitute BTS identity code (BSIC), transmitted on synchronous channel of each cell.
In many cases, different GSM PLMN employs the same frequency resource. But their network planning are independent from each other. To ensure that adjacent BTS with same frequency have different BSIC, it is generally regulated that adjacent GSM PLMN select different NCC.
Adjacent or close cells with the same BCCH frequency must have different BSIC. Special attention should be paid to the configuration of cells in boundary areas.