A sector is a “direction”. Usually, one sector = one cell.
In one BTS, you can have 3 cells, each covering one direction. So this BTS has 3 sectors.
In case of multiband network, you can have 2 cells per sector : 1 cell GSM900 and 1 cell DCS1800, both covering the same direction.
A sector has no “logical” existence in the OMC-R.