Difficulty in Understanding Mesh creation

Here are the contents of Meshmaker (This makes a radial grid with radius 224m and depth 10m, 

RZ2DL
RADII
    3
    0.0e00    5.0e-2    7.0e-2                                                   
LOGAR
   30          1.0e2    7.0e-2
RADII
    1
   5.001e2      
LAYER
    1
    1.0e01  

I fail to  understand why RADII
    1
   5.001e2  (500.1 ) is mentioned here, Wht is the logic through which this value is inputted in the meshmaker file?

someone please shed some light on this??