Possession is foundational to an understanding of the legal definition of slavery, even when the State does not support a property right in respect of persons. To determine, in law, a case of slavery, one must look for possession.
While the exact form of possession might vary, in essence it supposes control over a person by another such as a person might control a thing. Such control may be physical, but physical constraints will not always be necessary to the maintenance of effective control over a person. More abstract manifestations of control of a person may be evident in attempts to withhold identity documents; or to otherwise restrict free movement or access to state authorities or legal processes; or equally in attempts to forge a new identity through compelling a new religion, language, place of residence, or forcing marriage.
Fundamentally, where such control operates, it will significantly deprive that person of his or her individual liberty for a period of time which is, for that person, indeterminate.
Bellagio-Harvard Guidelines on the Legal Parameters of Slavery by the Members of the Research Network on the Legal Parameters of Slavery, March 2012