How does the diode on this display work?

[Starlord]

Page 9, one diode:
http://www.buydisplay.com/download/manual/ER-OLED015-1_Series_Datasheet.pdf

Page 54, two diodes:
http://www.buydisplay.com/download/ic/SSD1351.pdf

These diodes connect to VSL, which is described as a "segment voltage reference pin" for "voltage output low level".

I'm guessing this somehow sets the voltage that the display sends to the fets inside it when it wants to turn one of the pixels off.  But I don't understand why this wouldn't go straight to ground, or why one example has one diode and the other has two, and what the repercussions would be of having too few or too many diodes there.

I do know that my display worked with both one and two diodes. I didn't want to try no diodes and potentially blow the display up as they're $12 each.

Page 25 of the second PDF shows how a pixel is wired up if that helps.

[ataradov]

They are just used as more or less predictable voltage droppers. If you notice, on the first diagram they specify Vth =1.4V, and on the second one Vth = 0.7V, but there are two diodes. So the goal of that diode is to drop 1.4 V, and you need to select diode(s) accordingly.