[yt-users] What are the "x", "y" and "z" fields in a disk?

After defining a disk

my_disk=pf.h.disk(sc, [0,0,1], 450.0*pc, 30.0*pc)

I asked for the minimum and maximum values of these fields

xmin= my_disk["x"].min() xmax= my_disk["x"].max() ymin= my_disk["y"].min() ymax= my_disk["y"].max() zmin=my_disk["z"].min() zmax=my_disk["z"].max() print xmin,xmax,ymin,ymax,zmin,zmax

and got

0.00390625 0.99609375 0.00390625 0.99609375 0.00390625 0.99609375 xmin+xmax=ymin+ymax=zmin+zmax=1

So I really don't know what actually are these fields