Human trafficking. Or, modern slavery?

Human trafficking. Or, modern slavery? Politicians, United Nations, media and activists often use these phrases interchangeably. Yet, in recent years, there has been a subtle shift from the former to the latter. Legislative changes are evident in several countries (such as the recent modern slavery bill in the UK), and a range of organisations increasingly refer to modern slavery as opposed to human trafficking. This partial shift from trafficking to modern slavery has already been subject to critique. Yet, before even beginning debating the pros and cons of these concepts it is worth asking why this shift in language occurs in the first place. Why do we suddenly say modern slavery instead of trafficking?