THE TRUE SLAVE
They have something exceptional to offer: beauty, character, devout skills; the contract-slaves, or volunteers, also fall in this category (like Slave Nina in 'Fetish Ballerina').
When, over the years, you've become a true slave, and even your owner has become fond of your devotion and total submission. Most often, you'll serve a lifetime sentence as someone's property and the fused collar around your neck will probably accompany you in your grave. Give it a few more years and your owner might have become so fond of you, that they feel like you've served your sentence beyond the call of duty and, one day, you might be absolved and freed (though, that's not your goal any longer). These now domestic servants often continue to seek indenture, duty, and devotion.
ENSLAVED CONVICT
With mass incarceration, it's less personal. As you serve your many-year-long sentence as an indentured work slave, life is hard. You might get out in time if you don't break your back before your horrible sentence is over, or you might get further clauses added to your sentence. The overseers, who are either employed freewomen as lower mistresses, gynoid guards, or enforced through AI-controlled devices, are quick to add more years for laziness, bad behavior, showing attitude, escaping, resisting—you name it. And with those demerits, you will get more cruel slave gear permanently attached to your suffering body, which further dehumanizes you in front of everybody else (if you have to serve in public). Some slave sisters or brothers you'll never see again once they lost rank. It all serves to break your resistance and your mind, and it helps in enduring the years of slavehood—until you're reformed, free again, and absolved (at least, in the eyes of society). As a freedwoman or man, you have your sentence fully served now. (For men though, that can only happen in most of the surrounding neighbor states, and not in the Empire; they don't allow free men, only subservient male subjects who require collar and chains. And they all, in turn, become true slaves in households under an all-powerful Mistress.)
But wherever there is power in the hands of a few, there is abuse of said power and not all slaves are treated fairly, based on their merit. But generally, the system works and there is an old and continuous order. A freedwoman/man can find employment after, enjoy the luxuries of a slave-based society (like universal income, a clear hierarchy, well-funded government services,...) but can not afford slaves themselves; though, they get to enjoy the bright side now, in a highly archaic but-at-the-same-time technologically advanced society (like Bree in '
Slave Unit' does).