As said on this page https://gdal.org/drivers/vector/dxf.html,
GDAL writes DXF files with measurement units set to “Imperial - Inches”. If you need to change the units, edit the $MEASUREMENT and $INSUNITS variables in the header template
$MEASUREMENT and $INSUNITS are the variables in Autocad in order to choose between an imperial and a metric system.
As I never use the imperial system, is it possible to configure Qgis in order to systematically export vector layers in DXF format with a metric system? I precise that my Qgis is configured with the language, numbers, date and currency formats from the US).
I totally agree with the fact that this is not a big problem, but I have the feeling to lose so much time everyday just to change the unity from the imperial to the metric system in Autocad.
I think it would be interesting in the future to let that choice to the user in the "Save Vector Layer as ..." window...