I'm talking about ones that existed alongside the pre-civil war South, before they were driven out. It comes up as context in works that discuss a black slave fleeing to a nation where they're also a slave, but it's been a while since I read about that

That said, it's usually a safe bet that a system is better than or equal to black slavery in the US, because that perpetual, heritable, racialized chattel slavery is virtually an endpoint for the logic of slavery. Not all forms of slavery involve the following:

• Permanent slave status
• Slaves transferable and heritable as property
• Slave status for all descendents in perpetuity
• If not "owned", still designated as a slave by skin tone, to be captured and auctioned
• No family rights - married couples, parents, children all sold apart
• No rights at all, can be tortured or killed by owner

Most prominent systems of slavery haven't involved many or any of those points, but ours usee them all. You can even contrast this system with a more precedented one in the same country, for Irish sharecroppers specifically