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I think the 20mm 25mm 35mm etc he is referring to is the diameter of a scope mount ring.
Yes, the standard rail size for most (almost all) accessories is 20mm. That is the clamp that attaches to the rail. There is also an inner-diameter for mount rings that is dependent on the size of scope/RDS you are mounting.
For example, I use an Aimpoint RDS with a 40mm (I think) cantelever mount on a 20mm rail.
Hope that hasn't confused you further.
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