I really don’t know why they sent your file to Niagara Falls office. But I hope it’s not something negative, you have been approved so just wait for the medical request and other procedures. Atleast you got it , seems like they are working on May/June 2017 applications. I saw another thread where someone got a refusal who applied on June 2017. Which is sad. We all are in the same boat and waiting for something good to happen.
It was likely sent to Niagara Falls for the processing of the actual application for permanent residence. This would be medicals, security and criminal requirements. The initial decision was for a waiver to the requirement to apply from outside of Canada - Step 1, and Niagara Falls is Step 2. If you pass everything (Meds, Criminal and Security) then it is time to become a PR - Final Step
Fail any of those 3 statutory requirements then it is a whole other set of processing depending on what wasn't approved, as you might imagine. You are more than halfway home with processing since the waiver was approved, but how long this may take hinges on a number of things and you cannot really control these matters other than to remain in Canada, stay out of trouble, keep them informed about how to contact you; and be even more patient.
Celebrate quietly and be very satisfied with this decision.