Outside the extremes there is a significant overlap, though.

Historically, outside of race/religion/ethnicity based slavery (where slaves were generally kidnapped foreigners/outsiders or their descendants), it was a generally a continuum. Most societies were extremely hierarchical so there were various different tiers of unfree individuals who had different rights (or none at all if you were at the bottom) rather than some extremely clear boundary (like in North America). e.g. chattel slavery coexisted just fine with various types of serfdom in Early medieval Europe.

In many societies even "voluntary" slavery wasn't that uncommon because people were forced to chose between starving to death and giving up their (and usually their descendants) liberty.