Creating a new/clean masterfile

[the_iron_duke]

I'm still not sure of the steps needed to create a new/clean masterfile. Say, for example, I created a new unit. What would be all the things I'd need to delete from the old masterfile so they wouldn't affect a new game when importing the new masterfile? If the only change to a masterfile was the addition of a new unit, would I delete all other units, research and so on from the new masterfile, so all it contains is a new unit, or should all the default units and research be left on the new masterfile?