mm2, the common wire size in metric-speaking countries, rather than mm diameter, since I have had technical conversations with foreign engineers who used "mils" or "mm" as an abbreviation for area.
cross section area in mm2 is commonly used for high power electricity, such as mains lines, power cable between mains socket and high power equipment, etc. When you deal with more precise electronics, for example hand made RF inductors, transformers, in these area diameter in mm is commonly used...
For example bobbins with copper wire for inductors and transformers are marked with mm diameter. I don't remember bobbins which was marked with cross sectional area in mm^2.
I can't recall specifics, but when I look for info on wires, I seem to recall the use of terms which are not properly electronic. It's as if the electrical world tossed out all the info on units and just invented their own. So, I think there is a unit they use for area, which reads like a linear unit, circular mils, perhaps? This gets shorted to just mils in tables. Maybe that's also true for mm?
An American conventional unit (inch based) for wires is the "circular mil", which is defined as the area of a circle that is one mil in diameter, where one mil = 0.001 inch.
It is often found in design rules or rules-of-thumb for recommended current density in transformer windings, etc., in amperes per circular mil.
The American "AWG" gauges for wires, typical for "gauges", is a logarithmic measurement rather than linear measurement, where a difference of 3 in AWG is close to a factor of 2 in area: AWG 20 is 1,022 cir mil and AWG 17 is 2,048 cir mil in area.
My mnemonic is that AWG 10 is 0.1019 inch diameter, close to 1/10 inch.
For very large wires, larger than AWG 0 (sometimes called 1/0) = 0.3249 inch diameter or 106,000 cir mil area, the conventional series proceeds to multiple 0's: 00 (2/0), 000 (3/0), and 0000 (4/0).
Thereafter the amusing abbreviation "MCM" for "thousand circular mils" (as in the ancient Roman numeral M = 1000, not the modern mega).
I first ran into this when observing the installation of a new electrical box fed by "250 MCM" wire (0.5 inch diameter), the next size up from AWG 0000 at only 212,000 cir mil area.
Now, the skin depth of copper at 60 Hz is approximately 8.5 mm = 0.335 inch (depending on alloy), so one reaches diminishing returns in AC resistance per price of copper for solid copper wires above that diameter.
I believe that the maximum copper wire diameter in that series is 2000 MCM (1.414 in = 36 mm diameter).