sptool: nwind and nfft in Welch method spectrum viewer

It is unclear to me exactly what the Nfft and Nwind parameters are in the SPTool spectrum viewer when using the Welch method. Is it correct to assume Nfft is the same parameter as Nfft in the pwelch method and is also the length of the segment that the overall signal is divided up into? Is it correct to assume Nwind is the length of the window applied to the segment defined by Nfft? If so, what happens when and why would you specify a window shorter than the segment length?