The data center is composed of a 2'x2' grid, with the long axis marked in letters and the short axis marked in numbers. Alternatively, if the room is square, the wall opposite the main entry should be marked in letters.
Racks are referred to by grid coordinates based on the location of the left-front of the rack (usually, where the handle to open the front door is. That is, if you stand in front of the rack, the tile underneath the right front corner should is the rack's coordinates.