Diffusion grid pin on VFD display

[Rolo]

I'm exprimenting with a VFD display, I thought I knew the VFD principles but this model has a "Diffusion grid" pin. I have not seen that before. I suspect it has something to do with the size of the display, the digits are 15 mm high and it has only three filament wires.
It's the second pin from the left and it goes to the top glass with a little metal spring.
I have a section of the datasheet but that does not mention this pin, google does not help alot.
Anyone knows what the the function of this pin is and how to connect it? So far I have the best results if I connect it to the high voltage rail.

Thanks.

[Ian.M]

http://www.futabahk.com.hk/fts/-%20FTP_DataBase/Docs_05TechDocs/VFD/AN-1103A-EN%20%20VFD%20Characteristics%20and%20Operation%20Guide%20(EN).pdf

Page 13, section 6.5.

Tie it to the cathode bias voltage.  If that results in luminosity striping it may need to be taken positive via a resistor.

[Rolo]

Thanks!