I use "Lord" for any male noble without a position.
I also tend to use use Lord for someone whom it would be inappropriate to call simply "Sir", but who either has no current specific titles, or has so many title that it gets confusing to choose which title to use to address them. Elders of religions other than my own, for example.
The "proper" title for regional lords is baron or count or whichever is attached to their region, I don't think "Lord" is usually seen as strictly attached to the region lords.
