Drain to Gate Voltage in MOSFET

[masthdn]

Hi.

I use IRF820 in my project. (Vds=500V, Vdgr=500V, Vgs=20V).
What does mean the word "Vdgr"? Is it Maximum Drain to Gate voltage?
If yes, why does Vgs=20V but Vdg is not 20V?

 

[leg1234]

I think that IRF820 is VDMOS. Its structure is not sym.
You can refer to this doc.

http://www.irf.com/technical-info/appnotes/mosfet.pdf