The sectorid is a numeric value representing a sector in the universe. Currently it's ranging from 1 (Sol II A-1) to 7680 (Dantia P-16)
Given systemid (s), and coordinates (x, y), you could determine the sectorid mathematically using the following formula
sectorid = (s-1)*256 + (x-1)*16 + y
Take note that SplitSectorid returns a triple (systemid, sectorh, sectorv) with the coordinates swapped, so sectorh = y and sectorv = x.