What does it mean to "leave" the US? I know several people with advanced degrees in the sciences who had children in Europe and raised their kids there for year

What does it mean to "leave" the US? I know several people with advanced degrees in the sciences who had children in Europe and raised their kids there for years, but eventually came back. One family returned to Europe a second time. When you have a company or university sponsoring you it's not too difficult to move freely, so "leaving" feels like a moving target for this particular socioeconomic group.