There's another 2.9M Americans under probation, which rounds out the total to the 5.5M in the statistic.

We often hear about probation as a “lenient” sentence for someone facing possible time behind bars, and about parole as the act of a benevolent parole board, allowing an incarcerated person to be released before completing their maximum sentence. These one-dimensional views trick us into seeing these systems as entirely separate from incarceration, but they’re deeply linked. Staying in compliance with dozens of high-stakes, arbitrary rules is so unmanageable that experts call community supervision systems “a deprivation of liberty in their own right.” Moreover, probation and parole are often pathways into incarceration from the community, with violations of supervision accounting for 42% of prison admissions nationwide.

In practice, the process of probation outsources the costs associated with incarceration to the convict, while still imposing a litany of restrictions on travel, association, employment, and behavior through continuous surveillance. The difference between probation and incarceration largely comes down the second-order putative aspects of jail life (inedible food, lack of sleep, prison-encouraged social violence including murder). The primary purpose of jail - social isolation and continuous state monitoring - continues to be fulfilled.