Why PCB thickness in Microns
He is probably referring to copper layer thickness. He will probably start with FR4 plated by 18um Cu at both sides (start thickness) and eventually amends it with electroplating to 35 um (finish thickness), as he creates vias etc.

You probably need to specify the finish thickness only (depends on current density in the traces, 35um or 70um are the most common) and the manufacturer will then select the material that best suits his technology.