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