The locations of sectors within a galaxy are expressed using the Cartesian coordinate system. An (X,Y,Z) coordinate specifies the location of a sector by its distance from the center of the galaxy, measured in sectors: positive X is east; negative X is west; positive Y is north; negative Y is south; positive Z is up; negative Z is down.
For example, coordinate (312,-2,1) is 312 sectors east, then 2 sectors south, then 1 sector up from the center of the galaxy. The center of each galaxy is (0,0,0), which contains a supermassive black hole.
Sectors contain solar systems and large scale astronomical phenomena, such as nebulae.