Is substrate thickness specified in the Gerber file?

Hi,
I am trying to manufacture several 2-layer boards on FR4 with different
thicknesses.
0.8mm, 0.4mm etc.
My software (Eagle from Cadsoft) lets me specify substrate thickness in
the design rules.
However, I can't find any trace of the this info in the produced Gerber
files.
Is substrate thickness supposed to be in the gerbers, or supplied
separately?

Thanks,
Pavel

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