Classification of VDD and VSS pins
The types are used to check that the connections you make make sense. If you connect two inputs together without any other pins on that net the DRC should give you a warning. So think what you would allow and not allow for Vdd and Vss.  

I would assign Vdd as Power Input, since you can have several Power Inputs on the same net, with maximum 1 Power Output (from a voltage regulator, normally). For a moment I thought Vss could be passive, but that would allow you to connect an output to it, so that's not the right choice. Also Power Input for Vss, I think.