When do segments get recalculated?

When do segments get calculated?  For example:

1. When part of a campaign, do they get calculated when they are first added to the campaign? when the campaign is activated? and/or when the campaign actually runs (if you activate on a future date)? 
2. If I copy a campaign that has segments, do the segments get recalculated in the new campaign?  or do they stay at the numbers from the first campaign?

And if I want to freeze the segments, do I have to flatten them? Any other way?

Thanks everyone!