There is the entity that can use the SIN as an identifier through, I think, the Treasury Board Secretariat, and there are legal limitations as to who can ask and who we can ask to provide the SIN. It's the “don't” part that we advise the client on; don't give your SIN to people who ask for it except if they can legally do so and propose any other means to confirm your identity. People can provide viable licences or there are other kinds of means that they can use. But they should not provide the SIN except if the entity that requires the SIN can legally do so.
On April 1st, 2014. See this statement in context.