Actually it is mm2 [square millimeters]. About everything JD uses is metrics specs.
It is fun when it comes to tool sizes as most tool sets like to skip sizes and then you have order the sizes you need at an extra cost. Like they skip 15mm sockets in the impact sets and most of the 20mm-30 sizes. Even after years of buying tools I still run across a size I don't have. That throws a job off a week while I await the tool.
Just like this week I pulled out the deep well impact metric set to get a 15mm and was skipped. Luckily several years I needed the same socket so I dug thru the loose metric sockets and found it; although, the laser image is about gone but that is what an engraver is for.