I found this very old thread - I guess something could change in last 10 years, but “very small” torques are still in UE5.
Explanation is simple.
Torque unit in UE5 torque functions is 1/10000 Nm.
UE5 use cm (centimeter) as base measure for length, while SI use m (meter).
That implies that force unit in UE5 is not N (newton), but cN (centinewton) = 1/100N
Because 1N=1kg*1m/(s^2), co when we use cm as length unit we got cN as force.
Torque is forcedistance, in our case force is in cN, distance in cm.
So our unit of torque is kg(cm^2)/(s^2).
cm is squared! - that is why we need to scale it by 10000 to have SI unit of Nm.