Sourcing and sinking are not used consistently across manufacturers. My understanding is a sinking devices supplies common(NPN) and a sourcing device (PNP) supplies power. A sinking input requires a sourcing sensor and a sourcing output requires a sinking load. Pneumatic valves are often called sourcing but require a sourcing output (one that supplies power).
With that all said, in the US sinking inputs and sourcing outputs. I feel this is the best way because if a wire breaks and shorts to ground you cannot get a false input. If the output wire breaks and shorts to ground the load device will not be actuated. I have seen many times wires getting pinched and shorted to ground.
In Japan tends to be sourcing input and sinking output. Europe I believe is sinking inputs and sourcing outputs.