Apex only lists the consistent unit sets for bdf output, so mm-kg-s-N isn't shown.
The message is not well worded - it should say "a" custom units system, meaning any units system you've set up yourself.
Setting up a custom units system is how you can get it to write out inconsistent units. Use the "Clone" function to create a new system from the mm-kg-s-N system. Here you can tag the new cloned system as "Consistent" even though it isn't; now it should appear in the bdf output list.
Apex only lists the consistent unit sets for bdf output, so mm-kg-s-N isn't shown.
The message is not well worded - it should say "a" custom units system, meaning any units system you've set up yourself.
Setting up a custom units system is how you can get it to write out inconsistent units. Use the "Clone" function to create a new system from the mm-kg-s-N system. Here you can tag the new cloned system as "Consistent" even though it isn't; now it should appear in the bdf output list.
