When you're dealing with an adult, an adult can choose or can consent, but the difference between prostitution and trafficking is that it's not a question of consent; it's irrelevant. The person is being forced to provide their services, whether it's sexual or other labour, for an exploitative purpose, and they fear for their own safety or that of someone in their family if they don't provide those services. That's the distinction from prostitution. Prostitution is not illegal in Canada currently; it's the activities around prostitution that tend to be addressed through the Criminal Code. But there is that distinction.
I would emphasize the distinction in particular in dealing with children under the age of eighteen--which is what we've seen in the reported criminal cases, that they tend to be more under the age of eighteen--that it's never a question of consent.