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It does not matter too much of the unit you use, as the software can change between the two units.
You can use whatever you want.
I think mils is the most used unit.
It depends on the parts you're using and the capabilities of the
manufacturer. If the manufacturer is using mils, then you need to
use mil as unit.
While the metric system is arguably saner, some people are pretty adamant about using mils.
I think it has something to do with the machines used by manufacturers. So it depends on the manufacturer's prefer.
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