Chattel slavery is different from other forms of forced labor in that?

The slave is considered chattel - property - to be bought and sold in perpetuity like livestock. Once an individual becomes a chattel slave they are considered chattel for life. Not all forms of forced labor doom a person to slavery until the day they die. Many of them are only for a fixed period of time or until a fixed debt has been redeemed.

Children born to chattel slaves are considered the property of the "owner" of the chattel slave just like a calf is the property of the owner of the cow. No other form of forced labor passes that status/lifetime-obligation down to the children.