The constitution gives exclusive federal jurisdiction over employment as a component of its regulatory authority for specific industries, including banking, radio and TV broadcasting, inland and maritime navigation and shipping, inland and maritime fishing, as well as any form of transportation that crosses provincial boundaries (essentially aviation and rail transport but not highways). Employment outside of federally regulated industries falls under provincial authority for most civil (including contract) law.
The constitution gives federal authority over employment in certain industries (banking, radio/TV, etc.) while leaving most civil law matters to the provinces.