Ok, now I am confused - mL is a unit of volume, so how does it translate to thickness?

I'm fairly sure you mean mil here. 10mm would be over a third of an inch thick. That is a mistake that can be easily made though. Mililiter is abbreviated mL, and often shortened verbally to mil (especially if you spend alot of time in a chemistry lab). Because of that its easy to see mil on a plastic sheeting materal and convert that mil to mm. Not trying to be smart or anything, just wanting to make sure no beginners see that and get confused by it. I like the stuff too, Its usually sold as painters drop cloth for anyone interested in picking some up.