Does "Slaves" refer to "Prisoners" in the lock options? I thought the two meant the same but it seem like it does not.

My prisoners are not using the doors. I have them locked with the setting "Slaves Allowed" on. But I saw this thread and it seems like the only way to allow prisoners to use doors is to completely unlock the door. If that is true? If so what's the use for the "Slaves Allowed" option.